diff --git a/src/compiler/sourcemap.ts b/src/compiler/sourcemap.ts
index 8dfe91a6cff..dae4ddb45fd 100644
--- a/src/compiler/sourcemap.ts
+++ b/src/compiler/sourcemap.ts
@@ -156,7 +156,6 @@ namespace ts {
                 sourceMapNames: [],
                 sourceMapMappings: "",
                 sourceMapSourcesContent: compilerOptions.inlineSources ? [] : undefined,
-                sourceMapDecodedMappings: []
             };
 
             // Normalize source root and make sure it has trailing "/" so that it can be used to combine paths with the
@@ -299,7 +298,6 @@ namespace ts {
             }
 
             lastEncodedSourceMapSpan = lastRecordedSourceMapSpan;
-            sourceMapData.sourceMapDecodedMappings.push(lastEncodedSourceMapSpan);
         }
 
         /**
@@ -393,24 +391,29 @@ namespace ts {
 
                     const sourcesDirectoryPath = compilerOptions.sourceRoot ? host.getCommonSourceDirectory() : sourceMapDir;
                     const resolvedPathCache = createMap<string>();
-                    sourcemaps.calculateDecodedMappings(originalMap, (raw): void => {
+                    const absolutePathCache = createMap<string>();
+                    const sourcemapIterator = sourcemaps.decodeMappings(originalMap);
+                    for (let { value: raw, done } = sourcemapIterator.next(); !done; { value: raw, done } = sourcemapIterator.next()) {
+                        const pathCacheKey = "" + raw.sourceIndex;
                         // Apply offsets to each position and fixup source entries
-                        const rawPath = originalMap.sources[raw.sourceIndex];
-                        const relativePath = originalMap.sourceRoot ? combinePaths(originalMap.sourceRoot, rawPath) : rawPath;
-                        const combinedPath = combinePaths(getDirectoryPath(node.sourceMapPath!), relativePath);
-                        if (!resolvedPathCache.has(combinedPath)) {
-                            resolvedPathCache.set(combinedPath, getRelativePathToDirectoryOrUrl(
+                        if (!resolvedPathCache.has(pathCacheKey)) {
+                            const rawPath = originalMap.sources[raw.sourceIndex];
+                            const relativePath = originalMap.sourceRoot ? combinePaths(originalMap.sourceRoot, rawPath) : rawPath;
+                            const combinedPath = combinePaths(getDirectoryPath(node.sourceMapPath!), relativePath);
+                            const resolvedPath = getRelativePathToDirectoryOrUrl(
                                 sourcesDirectoryPath,
                                 combinedPath,
                                 host.getCurrentDirectory(),
                                 host.getCanonicalFileName,
                                 /*isAbsolutePathAnUrl*/ true
-                            ));
+                            );
+                            resolvedPathCache.set(pathCacheKey, resolvedPath);
+                            absolutePathCache.set(pathCacheKey, getNormalizedAbsolutePath(resolvedPath, sourcesDirectoryPath));
                         }
-                        const resolvedPath = resolvedPathCache.get(combinedPath)!;
-                        const absolutePath = getNormalizedAbsolutePath(resolvedPath, sourcesDirectoryPath);
+                        const resolvedPath = resolvedPathCache.get(pathCacheKey)!;
+                        const absolutePath = absolutePathCache.get(pathCacheKey)!;
                         // tslint:disable-next-line:no-null-keyword
-                        setupSourceEntry(absolutePath, originalMap.sourcesContent ? originalMap.sourcesContent[raw.sourceIndex] : null); // TODO: Lookup content for inlining?
+                        setupSourceEntry(absolutePath, originalMap.sourcesContent ? originalMap.sourcesContent[raw.sourceIndex] : null, resolvedPath); // TODO: Lookup content for inlining?
                         const newIndex = sourceMapData.sourceMapSources.indexOf(resolvedPath);
                         // Then reencode all the updated spans into the overall map
                         encodeLastRecordedSourceMapSpan();
@@ -420,7 +423,7 @@ namespace ts {
                             emittedColumn: raw.emittedLine === 0 ? (raw.emittedColumn + firstLineColumnOffset) : raw.emittedColumn,
                             sourceIndex: newIndex,
                         };
-                    });
+                    }
                     // And actually emit the text these sourcemaps are for
                     return emitCallback(hint, node);
                 }
@@ -519,17 +522,19 @@ namespace ts {
             setupSourceEntry(sourceFile.fileName, sourceFile.text);
         }
 
-        function setupSourceEntry(fileName: string, content: string | null) {
-            // Add the file to tsFilePaths
-            // If sourceroot option: Use the relative path corresponding to the common directory path
-            // otherwise source locations relative to map file location
-            const sourcesDirectoryPath = compilerOptions.sourceRoot ? host.getCommonSourceDirectory() : sourceMapDir;
+        function setupSourceEntry(fileName: string, content: string | null, source?: string) {
+            if (!source) {
+                // Add the file to tsFilePaths
+                // If sourceroot option: Use the relative path corresponding to the common directory path
+                // otherwise source locations relative to map file location
+                const sourcesDirectoryPath = compilerOptions.sourceRoot ? host.getCommonSourceDirectory() : sourceMapDir;
 
-            const source = getRelativePathToDirectoryOrUrl(sourcesDirectoryPath,
-                fileName,
-                host.getCurrentDirectory(),
-                host.getCanonicalFileName,
-                /*isAbsolutePathAnUrl*/ true);
+                source = getRelativePathToDirectoryOrUrl(sourcesDirectoryPath,
+                    fileName,
+                    host.getCurrentDirectory(),
+                    host.getCanonicalFileName,
+                    /*isAbsolutePathAnUrl*/ true);
+            }
 
             sourceMapSourceIndex = sourceMapData.sourceMapSources.indexOf(source);
             if (sourceMapSourceIndex === -1) {
diff --git a/src/compiler/sourcemapDecoder.ts b/src/compiler/sourcemapDecoder.ts
index 290f7699e5d..4248ac21829 100644
--- a/src/compiler/sourcemapDecoder.ts
+++ b/src/compiler/sourcemapDecoder.ts
@@ -190,7 +190,7 @@ namespace ts.sourcemaps {
         };
     }
 
-    export function calculateDecodedMappings<T>(map: SourceMapData, processPosition: (position: RawSourceMapPosition) => T, host?: { log?(s: string): void }): T[] {
+    function calculateDecodedMappings<T>(map: SourceMapData, processPosition: (position: RawSourceMapPosition) => T, host?: { log?(s: string): void }): T[] {
         const decoder = decodeMappings(map);
         const positions = arrayFrom(decoder, processPosition);
         if (decoder.error) {
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 403d6a46e03..5926b31ec41 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2844,7 +2844,6 @@ namespace ts {
         inputSourceFileNames: string[];      // Input source file (which one can use on program to get the file), 1:1 mapping with the sourceMapSources list
         sourceMapNames?: string[];           // Source map's names field - list of names that can be indexed in this source map
         sourceMapMappings: string;           // Source map's mapping field - encoded source map spans
-        sourceMapDecodedMappings: SourceMapSpan[];  // Raw source map spans that were encoded into the sourceMapMappings
     }
 
     /** Return code used by getEmitOutput function to indicate status of the function */
diff --git a/src/harness/documents.ts b/src/harness/documents.ts
index 52773bf774a..a1c75784f85 100644
--- a/src/harness/documents.ts
+++ b/src/harness/documents.ts
@@ -148,7 +148,7 @@ namespace documents {
 
         public static fromUrl(url: string) {
             const match = SourceMap._dataURLRegExp.exec(url);
-            return match ? new SourceMap(/*mapFile*/ undefined, new Buffer(match[1], "base64").toString("utf8")) : undefined;
+            return match ? new SourceMap(/*mapFile*/ undefined, (Buffer.from ? Buffer.from(match[1], "base64") : new Buffer(match[1], "base64")).toString("utf8")) : undefined;
         }
 
         public static fromSource(text: string): SourceMap | undefined {
diff --git a/src/harness/sourceMapRecorder.ts b/src/harness/sourceMapRecorder.ts
index 22f0659690b..a1aa07f8733 100644
--- a/src/harness/sourceMapRecorder.ts
+++ b/src/harness/sourceMapRecorder.ts
@@ -305,7 +305,8 @@ namespace Harness.SourceMapRecorder {
             }
 
             SourceMapSpanWriter.initializeSourceMapSpanWriter(sourceMapRecorder, sourceMapData, currentFile);
-            for (const decodedSourceMapping of sourceMapData.sourceMapDecodedMappings) {
+            const mapper = ts.sourcemaps.decodeMappings({ mappings: sourceMapData.sourceMapMappings, sources: sourceMapData.sourceMapSources });
+            for (let { value: decodedSourceMapping, done } = mapper.next(); !done; { value: decodedSourceMapping, done } = mapper.next()) {
                 const currentSourceFile = program.getSourceFile(sourceMapData.inputSourceFileNames[decodedSourceMapping.sourceIndex])!;
                 if (currentSourceFile !== prevSourceFile) {
                     SourceMapSpanWriter.recordNewSourceFileSpan(decodedSourceMapping, currentSourceFile.text);
diff --git a/src/harness/utils.ts b/src/harness/utils.ts
index d7e113c45fb..4650badadaf 100644
--- a/src/harness/utils.ts
+++ b/src/harness/utils.ts
@@ -63,10 +63,6 @@ namespace utils {
         return indentation;
     }
 
-    export function toUtf8(text: string): string {
-        return new Buffer(text).toString("utf8");
-    }
-
     export function getByteOrderMarkLength(text: string): number {
         if (text.length >= 1) {
             const ch0 = text.charCodeAt(0);
diff --git a/src/testRunner/parallel/host.ts b/src/testRunner/parallel/host.ts
index 988e181852a..c10be2a9ff8 100644
--- a/src/testRunner/parallel/host.ts
+++ b/src/testRunner/parallel/host.ts
@@ -20,6 +20,7 @@ namespace Harness.Parallel.Host {
         // tslint:disable-next-line:variable-name
         const FailedTestReporter = require(path.resolve(__dirname, "../../scripts/failed-tests")) as typeof import("../../../scripts/failed-tests");
 
+        const perfdataFileNameFragment = ".parallelperf";
         const perfData = readSavedPerfData(configOption);
         const newTasks: Task[] = [];
         let tasks: Task[] = [];
@@ -175,8 +176,6 @@ namespace Harness.Parallel.Host {
             }
         }
 
-        const perfdataFileNameFragment = ".parallelperf";
-
         function perfdataFileName(target?: string) {
             return `${perfdataFileNameFragment}${target ? `.${target}` : ""}.json`;
         }
diff --git a/src/testRunner/unittests/convertToBase64.ts b/src/testRunner/unittests/convertToBase64.ts
index e41f73034c4..ecf55d9855b 100644
--- a/src/testRunner/unittests/convertToBase64.ts
+++ b/src/testRunner/unittests/convertToBase64.ts
@@ -2,7 +2,7 @@ namespace ts {
     describe("convertToBase64", () => {
         function runTest(input: string): void {
             const actual = convertToBase64(input);
-            const expected = new Buffer(input).toString("base64");
+            const expected = (Buffer.from ? Buffer.from(input) : new Buffer(input)).toString("base64");
             assert.equal(actual, expected, "Encoded string using convertToBase64 does not match buffer.toString('base64')");
         }
 
diff --git a/src/tsserver/server.ts b/src/tsserver/server.ts
index cbb91ae70a8..0a70fb3d83b 100644
--- a/src/tsserver/server.ts
+++ b/src/tsserver/server.ts
@@ -199,7 +199,7 @@ namespace ts.server {
 
         private write(s: string) {
             if (this.fd >= 0) {
-                const buf = new Buffer(s);
+                const buf = Buffer.from ? Buffer.from(s) : new Buffer(s);
                 // tslint:disable-next-line no-null-keyword
                 fs.writeSync(this.fd, buf, 0, buf.length, /*position*/ null!); // TODO: GH#18217
             }
@@ -869,7 +869,7 @@ namespace ts.server {
     }
 
     // Override sys.write because fs.writeSync is not reliable on Node 4
-    sys.write = (s: string) => writeMessage(new Buffer(s, "utf8"));
+    sys.write = (s: string) => writeMessage(Buffer.from ? Buffer.from(s, "utf8") : new Buffer(s, "utf8"));
     sys.watchFile = (fileName, callback) => {
         const watchedFile = pollingWatchedFileSet.addFile(fileName, callback);
         return {
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index 612d1317597..240118283d3 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -2544,7 +2544,6 @@ declare namespace ts {
         inputSourceFileNames: string[];
         sourceMapNames?: string[];
         sourceMapMappings: string;
-        sourceMapDecodedMappings: SourceMapSpan[];
     }
     /** Return code used by getEmitOutput function to indicate status of the function */
     enum ExitStatus {
@@ -8604,17 +8603,6 @@ declare namespace ts.sourcemaps {
         readonly lastSpan: SourceMapSpan;
     }
     function decodeMappings(map: SourceMapData): MappingsDecoder;
-    function calculateDecodedMappings<T>(map: SourceMapData, processPosition: (position: RawSourceMapPosition) => T, host?: {
-        log?(s: string): void;
-    }): T[];
-    interface RawSourceMapPosition {
-        emittedLine: number;
-        emittedColumn: number;
-        sourceLine: number;
-        sourceColumn: number;
-        sourceIndex: number;
-        nameIndex?: number;
-    }
 }
 declare namespace ts {
     function getOriginalNodeId(node: Node): number;
diff --git a/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
index 60f44a00576..19cfd28043c 100644
--- a/tests/baselines/reference/api/typescript.d.ts
+++ b/tests/baselines/reference/api/typescript.d.ts
@@ -1822,7 +1822,6 @@ declare namespace ts {
         inputSourceFileNames: string[];
         sourceMapNames?: string[];
         sourceMapMappings: string;
-        sourceMapDecodedMappings: SourceMapSpan[];
     }
     /** Return code used by getEmitOutput function to indicate status of the function */
     enum ExitStatus {