diff --git a/build/lib/compilation.js b/build/lib/compilation.js
index 8407c7102f2..be9839af024 100644
--- a/build/lib/compilation.js
+++ b/build/lib/compilation.js
@@ -104,14 +104,19 @@ function compileTask(src, out, build) {
         // mangle: TypeScript to TypeScript
         let mangleStream = es.through();
         if (build) {
-            const ts2tsMangler = new mangleTypeScript_1.Mangler(compile.projectPath);
-            const replacer = ts2tsMangler.computeNewFileContents();
-            mangleStream = es.through(function (data) {
-                const newContents = replacer.get(data.path);
+            let ts2tsMangler = new mangleTypeScript_1.Mangler(compile.projectPath);
+            const newContentsByFileName = ts2tsMangler.computeNewFileContents();
+            mangleStream = es.through(function write(data) {
+                const newContents = newContentsByFileName.get(data.path);
                 if (newContents !== undefined) {
                     data.contents = Buffer.from(newContents);
                 }
                 this.push(data);
+            }, function end() {
+                this.push(null);
+                // free resources
+                newContentsByFileName.clear();
+                ts2tsMangler = undefined;
             });
         }
         return srcPipe
diff --git a/build/lib/compilation.ts b/build/lib/compilation.ts
index b1fb5879427..2cc04d2c977 100644
--- a/build/lib/compilation.ts
+++ b/build/lib/compilation.ts
@@ -123,14 +123,19 @@ export function compileTask(src: string, out: string, build: boolean): () => Nod
 		// mangle: TypeScript to TypeScript
 		let mangleStream = es.through();
 		if (build) {
-			const ts2tsMangler = new Mangler(compile.projectPath);
-			const replacer = ts2tsMangler.computeNewFileContents();
-			mangleStream = es.through(function (data: File) {
-				const newContents = replacer.get(data.path);
+			let ts2tsMangler = new Mangler(compile.projectPath);
+			const newContentsByFileName = ts2tsMangler.computeNewFileContents();
+			mangleStream = es.through(function write(data: File) {
+				const newContents = newContentsByFileName.get(data.path);
 				if (newContents !== undefined) {
 					data.contents = Buffer.from(newContents);
 				}
 				this.push(data);
+			}, function end() {
+				this.push(null);
+				// free resources
+				newContentsByFileName.clear();
+				(<any>ts2tsMangler) = undefined;
 			});
 		}
 
diff --git a/build/lib/mangleTypeScript.ts b/build/lib/mangleTypeScript.ts
index d767cbd4793..ab5198a2924 100644
--- a/build/lib/mangleTypeScript.ts
+++ b/build/lib/mangleTypeScript.ts
@@ -350,7 +350,7 @@ export class Mangler {
 		this.service = ts.createLanguageService(new StaticLanguageServiceHost(projectPath));
 	}
 
-	computeNewFileContents(): ReadonlyMap<string, string> {
+	computeNewFileContents(): Map<string, string> {
 
 		// STEP: find all classes and their field info