diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index 91a30143047..74b3f59bbd1 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -417,7 +417,7 @@ module ts {
         nodeIsNestedInLabel(label: Identifier, requireIterationStatement: boolean, stopAtFunctionBoundary: boolean): ControlBlockContext;
     }
 
-    export function createSourceFile(filename: string, sourceText: string, languageVersion: ScriptTarget, version: number = 0, isOpen: boolean = false): SourceFile {
+    export function createSourceFile(filename: string, sourceText: string, languageVersion: ScriptTarget, version: string, isOpen: boolean = false): SourceFile {
         var file: SourceFile;
         var scanner: Scanner;
         var token: SyntaxKind;
diff --git a/src/compiler/tsc.ts b/src/compiler/tsc.ts
index 054b76c55b2..85f9ce6fe32 100644
--- a/src/compiler/tsc.ts
+++ b/src/compiler/tsc.ts
@@ -142,7 +142,7 @@ module ts {
                 }
                 text = "";
             }
-            return text !== undefined ? createSourceFile(filename, text, languageVersion) : undefined;
+            return text !== undefined ? createSourceFile(filename, text, languageVersion, /*version:*/ "0") : undefined;
         }
 
         function writeFile(fileName: string, data: string, writeByteOrderMark: boolean, onError?: (message: string) => void) {
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 5cb80687d80..416a2960329 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -526,7 +526,7 @@ module ts {
         identifierCount: number;
         symbolCount: number;
         isOpen: boolean;
-        version: number;
+        version: string;
         languageVersion: ScriptTarget;
     }
 
diff --git a/src/harness/fourslash.ts b/src/harness/fourslash.ts
index da649f77dd9..fd91c7c70ec 100644
--- a/src/harness/fourslash.ts
+++ b/src/harness/fourslash.ts
@@ -1895,11 +1895,11 @@ module FourSlash {
         var result = '';
         var fourslashFilename = 'fourslash.ts';
         var tsFn = 'tests/cases/fourslash/' + fourslashFilename;
-        fourslashSourceFile = fourslashSourceFile || ts.createSourceFile(tsFn, Harness.IO.readFile(tsFn), ts.ScriptTarget.ES5, /*version*/ 0, /*isOpen*/ false);
+        fourslashSourceFile = fourslashSourceFile || ts.createSourceFile(tsFn, Harness.IO.readFile(tsFn), ts.ScriptTarget.ES5, /*version*/ "0", /*isOpen*/ false);
 
         var files: { [filename: string]: ts.SourceFile; } = {};
         files[fourslashFilename] = fourslashSourceFile;
-        files[fileName] = ts.createSourceFile(fileName, content, ts.ScriptTarget.ES5, /*version*/ 0, /*isOpen*/ false);
+        files[fileName] = ts.createSourceFile(fileName, content, ts.ScriptTarget.ES5, /*version*/ "0", /*isOpen*/ false);
         files[Harness.Compiler.defaultLibFileName] = Harness.Compiler.defaultLibSourceFile;
 
         var host = Harness.Compiler.createCompilerHost(files, (fn, contents) => result = contents);
diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index dce80544222..e19777827d1 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -532,7 +532,7 @@ module Harness {
         }
 
         export var defaultLibFileName = 'lib.d.ts';
-        export var defaultLibSourceFile = ts.createSourceFile(defaultLibFileName, IO.readFile(libFolder + 'lib.core.d.ts'), /*languageVersion*/ ts.ScriptTarget.ES5);
+        export var defaultLibSourceFile = ts.createSourceFile(defaultLibFileName, IO.readFile(libFolder + 'lib.core.d.ts'), /*languageVersion*/ ts.ScriptTarget.ES5, /*version:*/ "0");
 
         export function createCompilerHost(filemap: { [filename: string]: ts.SourceFile; }, writeFile: (fn: string, contents: string, writeByteOrderMark:boolean) => void): ts.CompilerHost {
             return {
@@ -724,7 +724,7 @@ module Harness {
                 var filemap: { [name: string]: ts.SourceFile; } = {};
                 var register = (file: { unitName: string; content: string; }) => {
                     var filename = Path.switchToForwardSlashes(file.unitName);
-                    filemap[filename] = ts.createSourceFile(filename, file.content, options.target);
+                    filemap[filename] = ts.createSourceFile(filename, file.content, options.target, /*version:*/ "0");
                 };
                 inputFiles.forEach(register);
                 otherFiles.forEach(register);
diff --git a/src/harness/harnessLanguageService.ts b/src/harness/harnessLanguageService.ts
index cb8ef61f413..0f904b24a17 100644
--- a/src/harness/harnessLanguageService.ts
+++ b/src/harness/harnessLanguageService.ts
@@ -80,8 +80,9 @@ module Harness.LanguageService {
             return JSON.stringify(this.lineMap.lineStarts());
         }
 
-        public getTextChangeRangeSinceVersion(scriptVersion: number): string {
-            var range = this.scriptInfo.getTextChangeRangeBetweenVersions(scriptVersion, this.version);
+        public getChangeRange(oldScript: ts.ScriptSnapshotShim): string {
+            var oldShim = <ScriptSnapshotShim>oldScript;
+            var range = this.scriptInfo.getTextChangeRangeBetweenVersions(oldShim.version, this.version);
             if (range === null) {
                 return null;
             }
@@ -101,50 +102,14 @@ module Harness.LanguageService {
         }
     }
 
-    class ScriptSnapshotShimAdapter implements TypeScript.IScriptSnapshot {
-        private lineStartPositions: number[] = null;
-        constructor(private scriptSnapshotShim: ts.ScriptSnapshotShim) {}
-        getText(start: number, end: number): string {return this.scriptSnapshotShim.getText(start, end);}
-        getLength(): number {return this.scriptSnapshotShim.getLength();}
-        getLineStartPositions(): number[] { return JSON.parse(this.scriptSnapshotShim.getLineStartPositions()); }
-        getTextChangeRangeSinceVersion(scriptVersion: number): TypeScript.TextChangeRange {
-            var encoded = this.scriptSnapshotShim.getTextChangeRangeSinceVersion(scriptVersion);
-            if (encoded == null) {
-                return null;
-            }
-
-            var decoded: { span: { start: number; length: number; }; newLength: number; } = JSON.parse(encoded);
-            return new TypeScript.TextChangeRange(
-                new TypeScript.TextSpan(decoded.span.start, decoded.span.length), decoded.newLength);
-        }
-    }
-
-    class LanguageServiceShimHostAdapter implements ts.LanguageServiceHost {
-        constructor(private shimHost: ts.LanguageServiceShimHost) { }
-        information(): boolean { return this.shimHost.information(); }
-        debug(): boolean { return this.shimHost.debug(); }
-        warning(): boolean { return this.shimHost.warning();}
-        error(): boolean { return this.shimHost.error(); }
-        fatal(): boolean { return this.shimHost.fatal(); }
-        log(s: string): void { this.shimHost.log(s); }
-        getCompilationSettings(): ts.CompilerOptions { return JSON.parse(this.shimHost.getCompilationSettings()); }
-        getScriptFileNames(): string[] { return JSON.parse(this.shimHost.getScriptFileNames());}
-        getScriptSnapshot(fileName: string): TypeScript.IScriptSnapshot { return new ScriptSnapshotShimAdapter(this.shimHost.getScriptSnapshot(fileName));}
-        getScriptVersion(fileName: string): number { return this.shimHost.getScriptVersion(fileName);}
-        getScriptIsOpen(fileName: string): boolean { return this.shimHost.getScriptIsOpen(fileName); }
-        getLocalizedDiagnosticMessages(): any { JSON.parse(this.shimHost.getLocalizedDiagnosticMessages());}
-        getCancellationToken(): ts.CancellationToken { return this.shimHost.getCancellationToken(); }
-    }
-
     export class NonCachingDocumentRegistry implements ts.DocumentRegistry {
-
         public static Instance: ts.DocumentRegistry = new NonCachingDocumentRegistry();
 
         public acquireDocument(
             fileName: string,
             compilationSettings: ts.CompilerOptions,
             scriptSnapshot: TypeScript.IScriptSnapshot,
-            version: number,
+            version: string,
             isOpen: boolean): ts.SourceFile {
             return ts.createSourceFile(fileName, scriptSnapshot.getText(0, scriptSnapshot.getLength()), compilationSettings.target, version, isOpen);
         }
@@ -154,7 +119,7 @@ module Harness.LanguageService {
             fileName: string,
             compilationSettings: ts.CompilerOptions,
             scriptSnapshot: TypeScript.IScriptSnapshot,
-            version: number,
+            version: string,
             isOpen: boolean,
             textChangeRange: TypeScript.TextChangeRange
             ): ts.SourceFile {
@@ -252,8 +217,8 @@ module Harness.LanguageService {
             return new ScriptSnapshotShim(this.getScriptInfo(fileName));
         }
 
-        public getScriptVersion(fileName: string): number {
-            return this.getScriptInfo(fileName).version;
+        public getScriptVersion(fileName: string): string {
+            return this.getScriptInfo(fileName).version.toString();
         }
 
         public getScriptIsOpen(fileName: string): boolean {
@@ -270,7 +235,7 @@ module Harness.LanguageService {
         public getLanguageService(): ts.LanguageServiceShim {
             var ls = new TypeScript.Services.TypeScriptServicesFactory().createLanguageServiceShim(this);
             this.ls = ls;
-            var hostAdapter = new LanguageServiceShimHostAdapter(this);
+            var hostAdapter = new ts.LanguageServiceShimHostAdapter(this);
 
             this.newLS = ts.createLanguageService(hostAdapter, NonCachingDocumentRegistry.Instance);
             return ls;
diff --git a/src/harness/projectsRunner.ts b/src/harness/projectsRunner.ts
index 5b4b1712e8e..c76fbda1b96 100644
--- a/src/harness/projectsRunner.ts
+++ b/src/harness/projectsRunner.ts
@@ -152,7 +152,7 @@ class ProjectRunner extends RunnerBase {
                     }
 
                     if (text !== undefined) {
-                        sourceFile = ts.createSourceFile(filename, text, languageVersion);
+                        sourceFile = ts.createSourceFile(filename, text, languageVersion, /*version:*/ "0");
                     }
                 }
 
diff --git a/src/harness/rwcRunner.ts b/src/harness/rwcRunner.ts
index 576b06e3435..780b6d24588 100644
--- a/src/harness/rwcRunner.ts
+++ b/src/harness/rwcRunner.ts
@@ -107,7 +107,7 @@ module RWC {
                             catch (e) {
                                 // Leave fileContents undefined;
                             }
-                            return ts.createSourceFile(fileName, fileContents, languageVersion);
+                            return ts.createSourceFile(fileName, fileContents, languageVersion, /*version:*/ "0");
                         },
                         getDefaultLibFilename: () => libPath,
                         writeFile: (fn, contents) => emitterIOHost.writeFile(fn, contents, false),
diff --git a/src/services/services.ts b/src/services/services.ts
index 06bfbd80e05..9ff6266d54e 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -26,7 +26,6 @@
 /// <reference path='compiler\pathUtils.ts' />
 
 module ts {
-
     export interface Node {
         getSourceFile(): SourceFile;
         getChildCount(): number;
@@ -72,7 +71,8 @@ module ts {
         getSourceUnit(): TypeScript.SourceUnitSyntax;
         getSyntaxTree(): TypeScript.SyntaxTree;
         getBloomFilter(): TypeScript.BloomFilter;
-        update(scriptSnapshot: TypeScript.IScriptSnapshot, version: number, isOpen: boolean, textChangeRange: TypeScript.TextChangeRange): SourceFile;
+        getScriptSnapshot(): TypeScript.IScriptSnapshot;
+        update(scriptSnapshot: TypeScript.IScriptSnapshot, version: string, isOpen: boolean, textChangeRange: TypeScript.TextChangeRange): SourceFile;
     }
 
     var scanner: Scanner = createScanner(ScriptTarget.ES5);
@@ -320,7 +320,7 @@ module ts {
         public identifierCount: number;
         public symbolCount: number;
         public statements: NodeArray<Statement>;
-        public version: number;
+        public version: string;
         public isOpen: boolean;
         public languageVersion: ScriptTarget;
 
@@ -333,6 +333,10 @@ module ts {
             return this.getSyntaxTree().sourceUnit();
         }
 
+        public getScriptSnapshot(): TypeScript.IScriptSnapshot {
+            return this.scriptSnapshot;
+        }
+
         public getLineMap(): TypeScript.LineMap {
             return this.getSyntaxTree().lineMap();
         }
@@ -384,7 +388,7 @@ module ts {
             return this.bloomFilter;
         }
 
-        public update(scriptSnapshot: TypeScript.IScriptSnapshot, version: number, isOpen: boolean, textChangeRange: TypeScript.TextChangeRange): SourceFile {
+        public update(scriptSnapshot: TypeScript.IScriptSnapshot, version: string, isOpen: boolean, textChangeRange: TypeScript.TextChangeRange): SourceFile {
             // See if we are currently holding onto a syntax tree.  We may not be because we're 
             // either a closed file, or we've just been lazy and haven't had to create the syntax
             // tree yet.  Access the field instead of the method so we don't accidently realize
@@ -419,7 +423,7 @@ module ts {
             return SourceFileObject.createSourceFileObject(this.filename, scriptSnapshot, this.languageVersion, version, isOpen, newSyntaxTree);
         }
 
-        public static createSourceFileObject(filename: string, scriptSnapshot: TypeScript.IScriptSnapshot, languageVersion: ScriptTarget, version: number, isOpen: boolean, syntaxTree?: TypeScript.SyntaxTree) {
+        public static createSourceFileObject(filename: string, scriptSnapshot: TypeScript.IScriptSnapshot, languageVersion: ScriptTarget, version: string, isOpen: boolean, syntaxTree?: TypeScript.SyntaxTree) {
             var newSourceFile = <SourceFileObject><any>createSourceFile(filename, scriptSnapshot.getText(0, scriptSnapshot.getLength()), languageVersion, version, isOpen);
             newSourceFile.scriptSnapshot = scriptSnapshot;
             newSourceFile.syntaxTree = syntaxTree;
@@ -442,7 +446,7 @@ module ts {
     export interface LanguageServiceHost extends Logger {
         getCompilationSettings(): CompilerOptions;
         getScriptFileNames(): string[];
-        getScriptVersion(fileName: string): number;
+        getScriptVersion(fileName: string): string;
         getScriptIsOpen(fileName: string): boolean;
         getScriptSnapshot(fileName: string): TypeScript.IScriptSnapshot;
         getLocalizedDiagnosticMessages(): any;
@@ -692,7 +696,7 @@ module ts {
             filename: string,
             compilationSettings: CompilerOptions,
             scriptSnapshot: TypeScript.IScriptSnapshot,
-            version: number,
+            version: string,
             isOpen: boolean,
             referencedFiles: string[]): SourceFile;
 
@@ -701,7 +705,7 @@ module ts {
             filename: string,
             compilationSettings: CompilerOptions,
             scriptSnapshot: TypeScript.IScriptSnapshot,
-            version: number,
+            version: string,
             isOpen: boolean,
             textChangeRange: TypeScript.TextChangeRange
             ): SourceFile;
@@ -819,7 +823,7 @@ module ts {
     // Information about a specific host file.
     interface HostFileInformation {
         filename: string;
-        version: number;
+        version: string;
         isOpen: boolean;
         sourceText?: TypeScript.IScriptSnapshot;
     }
@@ -928,7 +932,7 @@ module ts {
             return fileNames;
         }
 
-        public getVersion(filename: string): number {
+        public getVersion(filename: string): string {
             return this.getEntry(filename).version;
         }
 
@@ -944,14 +948,14 @@ module ts {
             return file.sourceText;
         }
 
-        public getScriptTextChangeRangeSinceVersion(filename: string, lastKnownVersion: number): TypeScript.TextChangeRange {
+        public getChangeRange(filename: string, lastKnownVersion: string, oldScriptSnapshot: TypeScript.IScriptSnapshot): TypeScript.TextChangeRange {
             var currentVersion = this.getVersion(filename);
             if (lastKnownVersion === currentVersion) {
                 return TypeScript.TextChangeRange.unchanged; // "No changes"
             }
 
             var scriptSnapshot = this.getScriptSnapshot(filename);
-            return scriptSnapshot.getTextChangeRangeSinceVersion(lastKnownVersion);
+            return scriptSnapshot.getChangeRange(oldScriptSnapshot);
         }
     }
 
@@ -960,11 +964,10 @@ module ts {
 
         // For our syntactic only features, we also keep a cache of the syntax tree for the 
         // currently edited file.  
-        private currentfilename: string = "";
-        private currentFileVersion: number = -1;
+        private currentFilename: string = "";
+        private currentFileVersion: string = null;
         private currentSourceFile: SourceFile = null;
         private currentFileSyntaxTree: TypeScript.SyntaxTree = null;
-        private currentFileScriptSnapshot: TypeScript.IScriptSnapshot = null;
 
         constructor(private host: LanguageServiceHost) {
             this.hostCache = new HostCache(host);
@@ -979,7 +982,7 @@ module ts {
             var syntaxTree: TypeScript.SyntaxTree = null;
             var sourceFile: SourceFile;
 
-            if (this.currentFileSyntaxTree === null || this.currentfilename !== filename) {
+            if (this.currentFileSyntaxTree === null || this.currentFilename !== filename) {
                 var scriptSnapshot = this.hostCache.getScriptSnapshot(filename);
                 syntaxTree = this.createSyntaxTree(filename, scriptSnapshot);
                 sourceFile = createSourceFileFromScriptSnapshot(filename, scriptSnapshot, getDefaultCompilerOptions(), version, /*isOpen*/ true);
@@ -988,9 +991,10 @@ module ts {
             }
             else if (this.currentFileVersion !== version) {
                 var scriptSnapshot = this.hostCache.getScriptSnapshot(filename);
-                syntaxTree = this.updateSyntaxTree(filename, scriptSnapshot, this.currentFileSyntaxTree, this.currentFileVersion);
+                syntaxTree = this.updateSyntaxTree(filename, scriptSnapshot,
+                    this.currentSourceFile.getScriptSnapshot(), this.currentFileSyntaxTree, this.currentFileVersion);
 
-                var editRange = this.hostCache.getScriptTextChangeRangeSinceVersion(filename, this.currentFileVersion);
+                var editRange = this.hostCache.getChangeRange(filename, this.currentFileVersion, this.currentSourceFile.getScriptSnapshot());
                 sourceFile = !editRange 
                     ? createSourceFileFromScriptSnapshot(filename, scriptSnapshot, getDefaultCompilerOptions(), version, /*isOpen*/ true)
                     : this.currentSourceFile.update(scriptSnapshot, version, /*isOpen*/ true, editRange);
@@ -1001,9 +1005,8 @@ module ts {
             if (syntaxTree !== null) {
                 Debug.assert(sourceFile);
                 // All done, ensure state is up to date
-                this.currentFileScriptSnapshot = scriptSnapshot;
                 this.currentFileVersion = version;
-                this.currentfilename = filename;
+                this.currentFilename = filename;
                 this.currentFileSyntaxTree = syntaxTree;
                 this.currentSourceFile = sourceFile;
             }
@@ -1038,7 +1041,7 @@ module ts {
         public getCurrentScriptSnapshot(filename: string): TypeScript.IScriptSnapshot {
             // update currentFileScriptSnapshot as a part of 'getCurrentFileSyntaxTree' call
             this.getCurrentFileSyntaxTree(filename);
-            return this.currentFileScriptSnapshot;
+            return this.getCurrentSourceFile(filename).getScriptSnapshot();
         }
 
         private createSyntaxTree(filename: string, scriptSnapshot: TypeScript.IScriptSnapshot): TypeScript.SyntaxTree {
@@ -1052,8 +1055,8 @@ module ts {
             return syntaxTree;
         }
 
-        private updateSyntaxTree(filename: string, scriptSnapshot: TypeScript.IScriptSnapshot, previousSyntaxTree: TypeScript.SyntaxTree, previousFileVersion: number): TypeScript.SyntaxTree {
-            var editRange = this.hostCache.getScriptTextChangeRangeSinceVersion(filename, previousFileVersion);
+        private updateSyntaxTree(filename: string, scriptSnapshot: TypeScript.IScriptSnapshot, previousScriptSnapshot: TypeScript.IScriptSnapshot, previousSyntaxTree: TypeScript.SyntaxTree, previousFileVersion: string): TypeScript.SyntaxTree {
+            var editRange = this.hostCache.getChangeRange(filename, previousFileVersion, previousScriptSnapshot);
 
             // Debug.assert(newLength >= 0);
 
@@ -1065,7 +1068,7 @@ module ts {
             var nextSyntaxTree = TypeScript.IncrementalParser.parse(
                 previousSyntaxTree, editRange, TypeScript.SimpleText.fromScriptSnapshot(scriptSnapshot));
 
-            this.ensureInvariants(filename, editRange, nextSyntaxTree, this.currentFileScriptSnapshot, scriptSnapshot);
+            this.ensureInvariants(filename, editRange, nextSyntaxTree, previousScriptSnapshot, scriptSnapshot);
 
             return nextSyntaxTree;
         }
@@ -1133,7 +1136,7 @@ module ts {
         }
     }
 
-    function createSourceFileFromScriptSnapshot(filename: string, scriptSnapshot: TypeScript.IScriptSnapshot, settings: CompilerOptions, version: number, isOpen: boolean) {
+    function createSourceFileFromScriptSnapshot(filename: string, scriptSnapshot: TypeScript.IScriptSnapshot, settings: CompilerOptions, version: string, isOpen: boolean) {
         return SourceFileObject.createSourceFileObject(filename, scriptSnapshot, settings.target, version, isOpen);
     }
 
@@ -1178,7 +1181,7 @@ module ts {
             filename: string,
             compilationSettings: CompilerOptions,
             scriptSnapshot: TypeScript.IScriptSnapshot,
-            version: number,
+            version: string,
             isOpen: boolean): SourceFile {
 
             var bucket = getBucketForCompilationSettings(compilationSettings, /*createIfMissing*/ true);
@@ -1202,7 +1205,7 @@ module ts {
             filename: string,
             compilationSettings: CompilerOptions,
             scriptSnapshot: TypeScript.IScriptSnapshot,
-            version: number,
+            version: string,
             isOpen: boolean,
             textChangeRange: TypeScript.TextChangeRange
             ): SourceFile {
@@ -1388,7 +1391,7 @@ module ts {
                     // new text buffer).
                     var textChangeRange: TypeScript.TextChangeRange = null;
                     if (sourceFile.isOpen && isOpen) {
-                        textChangeRange = hostCache.getScriptTextChangeRangeSinceVersion(filename, sourceFile.version);
+                        textChangeRange = hostCache.getChangeRange(filename, sourceFile.version, sourceFile.getScriptSnapshot());
                     }
 
                     sourceFile = documentRegistry.updateDocument(sourceFile, filename, compilationSettings, scriptSnapshot, version, isOpen, textChangeRange);
diff --git a/src/services/shims.ts b/src/services/shims.ts
index 523a0af5c91..d473a6979ee 100644
--- a/src/services/shims.ts
+++ b/src/services/shims.ts
@@ -36,7 +36,7 @@ module ts {
         //  { span: { start: number; length: number }; newLength: number }
         //
         // Or null value if there was no change.
-        getTextChangeRangeSinceVersion(scriptVersion: number): string;
+        getChangeRange(oldSnapshot: ScriptSnapshotShim): string;
     }
 
     //
@@ -48,7 +48,7 @@ module ts {
         // Returns a JSON encoded value of the type:
         // string[]
         getScriptFileNames(): string;
-        getScriptVersion(fileName: string): number;
+        getScriptVersion(fileName: string): string;
         getScriptIsOpen(fileName: string): boolean;
         getScriptSnapshot(fileName: string): ScriptSnapshotShim;
         getLocalizedDiagnosticMessages(): string;
@@ -283,8 +283,9 @@ module ts {
             return this.lineStartPositions;
         }
 
-        public getTextChangeRangeSinceVersion(scriptVersion: number): TypeScript.TextChangeRange {
-            var encoded = this.scriptSnapshotShim.getTextChangeRangeSinceVersion(scriptVersion);
+        public getChangeRange(oldSnapshot: TypeScript.IScriptSnapshot): TypeScript.TextChangeRange {
+            var oldSnapshotShim = <ScriptSnapshotShimAdapter>oldSnapshot;
+            var encoded = this.scriptSnapshotShim.getChangeRange(oldSnapshotShim.scriptSnapshotShim);
             if (encoded == null) {
                 return null;
             }
@@ -295,7 +296,7 @@ module ts {
         }
     }
 
-    class LanguageServiceShimHostAdapter implements LanguageServiceHost {
+    export class LanguageServiceShimHostAdapter implements LanguageServiceHost {
         constructor(private shimHost: LanguageServiceShimHost) {
         }
 
@@ -349,7 +350,7 @@ module ts {
             return new ScriptSnapshotShimAdapter(this.shimHost.getScriptSnapshot(fileName));
         }
 
-        public getScriptVersion(fileName: string): number {
+        public getScriptVersion(fileName: string): string {
             return this.shimHost.getScriptVersion(fileName);
         }
 
diff --git a/src/services/text/scriptSnapshot.ts b/src/services/text/scriptSnapshot.ts
index 41b5ed37c41..1c7091d6cae 100644
--- a/src/services/text/scriptSnapshot.ts
+++ b/src/services/text/scriptSnapshot.ts
@@ -16,10 +16,12 @@ module TypeScript {
         // always determine this (albeit in a more expensive manner).
         getLineStartPositions(): number[];
 
-        // Returns a text change range representing what text has changed since the specified version.
-        // If the change cannot be determined (say, because a file was opened/closed), then 'null' 
-        // should be returned.
-        getTextChangeRangeSinceVersion(scriptVersion: number): TextChangeRange;
+        // Gets the TextChangeRange that describe how the text changed between this text and 
+        // an older version.  This informatoin is used by the incremental parser to determine
+        // what sections of the script need to be reparsed.  'null' can be returned if the 
+        // change range cannot be determined.  However, in that case, incremental parsing will
+        // not happen and the entire document will be reparsed.
+        getChangeRange(oldSnapshot: IScriptSnapshot): TextChangeRange;
     }
 
     export module ScriptSnapshot {
@@ -45,7 +47,7 @@ module TypeScript {
                 return this._lineStartPositions;
             }
 
-            public getTextChangeRangeSinceVersion(scriptVersion: number): TypeScript.TextChangeRange {
+            public getChangeRange(oldSnapshot: IScriptSnapshot): TextChangeRange {
                 throw Errors.notYetImplemented();
             }
         }