diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index f11c88b2a0b..5bb5142d570 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -417,8 +417,8 @@ namespace ts {
     // from this SourceFile that are being held onto may change as a result (including
     // becoming detached from any SourceFile).  It is recommended that this SourceFile not
     // be used once 'update' is called on it.
-    export function updateSourceFile(sourceFile: SourceFile, newText: string, textChangeRange: TextChangeRange, aggressiveChecks?: boolean, scriptKind?: ScriptKind): SourceFile {
-        return IncrementalParser.updateSourceFile(sourceFile, newText, textChangeRange, aggressiveChecks, scriptKind);
+    export function updateSourceFile(sourceFile: SourceFile, newText: string, textChangeRange: TextChangeRange, aggressiveChecks?: boolean): SourceFile {
+        return IncrementalParser.updateSourceFile(sourceFile, newText, textChangeRange, aggressiveChecks);
     }
 
     /* @internal */
@@ -536,11 +536,10 @@ namespace ts {
 
         export function parseSourceFile(fileName: string, _sourceText: string, languageVersion: ScriptTarget, _syntaxCursor: IncrementalParser.SyntaxCursor, setParentNodes?: boolean, scriptKind?: ScriptKind): SourceFile {
 
-            const isJavaScriptFile = hasJavaScriptFileExtension(fileName) || _sourceText.lastIndexOf("// @language=javascript", 0) === 0 || scriptKind == ScriptKind.JS;
-
+            const isJavaScriptFile = scriptKind === ScriptKind.JS || hasJavaScriptFileExtension(fileName);
             initializeState(fileName, _sourceText, languageVersion, isJavaScriptFile, _syntaxCursor);
 
-            const result = parseSourceFileWorker(fileName, languageVersion, setParentNodes);
+            const result = parseSourceFileWorker(fileName, languageVersion, setParentNodes, scriptKind);
 
             clearState();
 
@@ -588,8 +587,8 @@ namespace ts {
             sourceText = undefined;
         }
 
-        function parseSourceFileWorker(fileName: string, languageVersion: ScriptTarget, setParentNodes: boolean): SourceFile {
-            sourceFile = createSourceFile(fileName, languageVersion);
+        function parseSourceFileWorker(fileName: string, languageVersion: ScriptTarget, setParentNodes: boolean, scriptKind?: ScriptKind): SourceFile {
+            sourceFile = createSourceFile(fileName, languageVersion, scriptKind);
             sourceFile.flags = contextFlags;
 
             // Prime the scanner.
@@ -656,7 +655,7 @@ namespace ts {
             }
         }
 
-        function createSourceFile(fileName: string, languageVersion: ScriptTarget): SourceFile {
+        function createSourceFile(fileName: string, languageVersion: ScriptTarget, scriptKind?: ScriptKind): SourceFile {
             // code from createNode is inlined here so createNode won't have to deal with special case of creating source files
             // this is quite rare comparing to other nodes and createNode should be as fast as possible
             const sourceFile = <SourceFile>new SourceFileConstructor(SyntaxKind.SourceFile, /*pos*/ 0, /* end */ sourceText.length);
@@ -669,6 +668,9 @@ namespace ts {
             sourceFile.languageVariant = getLanguageVariant(sourceFile.fileName);
             sourceFile.isDeclarationFile = fileExtensionIs(sourceFile.fileName, ".d.ts");
 
+            scriptKind = scriptKind || ScriptKind.Unknown;
+            sourceFile.scriptKind = scriptKind !== ScriptKind.Unknown ? scriptKind : ScriptKind.TS;
+
             return sourceFile;
         }
 
@@ -6225,7 +6227,7 @@ namespace ts {
     }
 
     namespace IncrementalParser {
-        export function updateSourceFile(sourceFile: SourceFile, newText: string, textChangeRange: TextChangeRange, aggressiveChecks: boolean, scriptKind?: ScriptKind): SourceFile {
+        export function updateSourceFile(sourceFile: SourceFile, newText: string, textChangeRange: TextChangeRange, aggressiveChecks: boolean): SourceFile {
             aggressiveChecks = aggressiveChecks || Debug.shouldAssert(AssertionLevel.Aggressive);
 
             checkChangeRange(sourceFile, newText, textChangeRange, aggressiveChecks);
@@ -6237,7 +6239,7 @@ namespace ts {
             if (sourceFile.statements.length === 0) {
                 // If we don't have any statements in the current source file, then there's no real
                 // way to incrementally parse.  So just do a full parse instead.
-                return Parser.parseSourceFile(sourceFile.fileName, newText, sourceFile.languageVersion, /*syntaxCursor*/ undefined, /*setParentNodes*/ true, scriptKind);
+                return Parser.parseSourceFile(sourceFile.fileName, newText, sourceFile.languageVersion, /*syntaxCursor*/ undefined, /*setParentNodes*/ true, sourceFile.scriptKind);
             }
 
             // Make sure we're not trying to incrementally update a source file more than once.  Once
@@ -6301,7 +6303,7 @@ namespace ts {
             // inconsistent tree.  Setting the parents on the new tree should be very fast.  We
             // will immediately bail out of walking any subtrees when we can see that their parents
             // are already correct.
-            const result = Parser.parseSourceFile(sourceFile.fileName, newText, sourceFile.languageVersion, syntaxCursor, /*setParentNodes*/ true, scriptKind);
+            const result = Parser.parseSourceFile(sourceFile.fileName, newText, sourceFile.languageVersion, syntaxCursor, /*setParentNodes*/ true, sourceFile.scriptKind);
 
             return result;
         }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 8c9d1d1d40a..2b76f8a57fb 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -1532,6 +1532,7 @@ namespace ts {
         hasNoDefaultLib: boolean;
 
         languageVersion: ScriptTarget;
+        /* @internal */ scriptKind: ScriptKind;
 
         // The first node that causes this file to be an external module
         /* @internal */ externalModuleIndicator: Node;
diff --git a/src/services/services.ts b/src/services/services.ts
index 80bb0baf629..cb5168039b9 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -806,6 +806,7 @@ namespace ts {
         public identifierCount: number;
         public symbolCount: number;
         public version: string;
+        public scriptKind: ScriptKind;
         public languageVersion: ScriptTarget;
         public languageVariant: LanguageVariant;
         public identifiers: Map<string>;
@@ -819,8 +820,8 @@ namespace ts {
             super(kind, pos, end);
         }
 
-        public update(newText: string, textChangeRange: TextChangeRange, scriptKind?: ScriptKind): SourceFile {
-            return updateSourceFile(this, newText, textChangeRange, /*aggressiveChecks*/ undefined, scriptKind);
+        public update(newText: string, textChangeRange: TextChangeRange): SourceFile {
+            return updateSourceFile(this, newText, textChangeRange);
         }
 
         public getLineAndCharacterOfPosition(position: number): LineAndCharacter {
@@ -1834,7 +1835,7 @@ namespace ts {
             else if (this.currentFileVersion !== version) {
                 // This is the same file, just a newer version. Incrementally parse the file.
                 const editRange = scriptSnapshot.getChangeRange(this.currentFileScriptSnapshot);
-                sourceFile = updateLanguageServiceSourceFile(this.currentSourceFile, scriptSnapshot, version, editRange, /*aggressiveChecks*/ undefined, scriptKind);
+                sourceFile = updateLanguageServiceSourceFile(this.currentSourceFile, scriptSnapshot, version, editRange);
             }
 
             if (sourceFile) {
@@ -1969,7 +1970,7 @@ namespace ts {
 
     export let disableIncrementalParsing = false;
 
-    export function updateLanguageServiceSourceFile(sourceFile: SourceFile, scriptSnapshot: IScriptSnapshot, version: string, textChangeRange: TextChangeRange, aggressiveChecks?: boolean, scriptKind?: ScriptKind): SourceFile {
+    export function updateLanguageServiceSourceFile(sourceFile: SourceFile, scriptSnapshot: IScriptSnapshot, version: string, textChangeRange: TextChangeRange, aggressiveChecks?: boolean): SourceFile {
         // If we were given a text change range, and our version or open-ness changed, then
         // incrementally parse this file.
         if (textChangeRange) {
@@ -2003,7 +2004,7 @@ namespace ts {
                                 : (changedText + suffix);
                     }
 
-                    const newSourceFile = updateSourceFile(sourceFile, newText, textChangeRange, aggressiveChecks, scriptKind);
+                    const newSourceFile = updateSourceFile(sourceFile, newText, textChangeRange, aggressiveChecks);
                     setSourceFileFields(newSourceFile, scriptSnapshot, version);
                     // after incremental parsing nameTable might not be up-to-date
                     // drop it so it can be lazily recreated later
@@ -2024,7 +2025,7 @@ namespace ts {
         }
 
         // Otherwise, just create a new source file.
-        return createLanguageServiceSourceFile(sourceFile.fileName, scriptSnapshot, sourceFile.languageVersion, version, /*setNodeParents*/ true, scriptKind);
+        return createLanguageServiceSourceFile(sourceFile.fileName, scriptSnapshot, sourceFile.languageVersion, version, /*setNodeParents*/ true, sourceFile.scriptKind);
     }
 
     export function createDocumentRegistry(useCaseSensitiveFileNames?: boolean, currentDirectory = ""): DocumentRegistry {
@@ -2104,7 +2105,7 @@ namespace ts {
                 // return it as is.
                 if (entry.sourceFile.version !== version) {
                     entry.sourceFile = updateLanguageServiceSourceFile(entry.sourceFile, scriptSnapshot, version,
-                        scriptSnapshot.getChangeRange(entry.sourceFile.scriptSnapshot), /*aggressiveChecks*/ undefined, scriptKind);
+                        scriptSnapshot.getChangeRange(entry.sourceFile.scriptSnapshot));
                 }
             }
 
diff --git a/src/services/shims.ts b/src/services/shims.ts
index 571c75482bb..f8c51feca27 100644
--- a/src/services/shims.ts
+++ b/src/services/shims.ts
@@ -348,10 +348,10 @@ namespace ts {
         }
 
         public getScriptKind(fileName: string): ScriptKind {
-            try {
+            if ("getScriptKind" in this.shimHost) {
                 return this.shimHost.getScriptKind(fileName);
             }
-            catch (e) {
+            else {
                 return ScriptKind.Unknown;
             }
         }