diff --git a/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer.ts b/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer.ts
index 3095f1eb67..ed9ddfbca5 100644
--- a/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer.ts
+++ b/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer.ts
@@ -19,6 +19,7 @@ import { getScrubFileTransformer } from '../transforms/scrub-file';
 const hasDecorators = /decorators/;
 const hasCtorParameters = /ctorParameters/;
 const hasTsHelpers = /var (__extends|__decorate|__metadata|__param) = /;
+const isAngularPackage = /(\\|\/)node_modules(\\|\/)@angular(\\|\/)/;
 
 export interface BuildOptimizerOptions {
   content?: string;
@@ -50,15 +51,22 @@ export function buildOptimizer(options: BuildOptimizerOptions):
     getTransforms.push(getImportTslibTransformer);
   }
 
-
-  if (hasDecorators.test(content) || hasCtorParameters.test(content)) {
+  if (inputFilePath && isAngularPackage.test(inputFilePath)) {
     // Order matters, getPrefixFunctionsTransformer needs to be called before
     // getFoldFileTransformer.
-    getTransforms.push(...[
+    getTransforms.push(
+      // getPrefixFunctionsTransformer is rather dangerous.
+      // It will mark both `require()` calls and `console.log(stuff)` as pure.
+      // We only apply it to @angular/* packages, since we know they are safe.
       getPrefixFunctionsTransformer,
       getScrubFileTransformer,
       getFoldFileTransformer,
-    ]);
+    );
+  } else if (hasDecorators.test(content) || hasCtorParameters.test(content)) {
+    getTransforms.push(
+      getScrubFileTransformer,
+      getFoldFileTransformer,
+    );
   }
 
   if (getTransforms.length > 0) {
diff --git a/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer_spec.ts b/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer_spec.ts
index c7cc3f7e9c..560f60b7a7 100644
--- a/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer_spec.ts
+++ b/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer_spec.ts
@@ -40,10 +40,11 @@ describe('build-optimizer', () => {
       `;
       // tslint:enable:max-line-length
 
-      expect(oneLine`${buildOptimizer({ content: input }).content}`).toEqual(output);
+      const inputFilePath = '/node_modules/@angular/some-lib';
+      expect(oneLine`${buildOptimizer({ content: input, inputFilePath }).content}`).toEqual(output);
     });
 
-    it('doesn\'t process files without decorators/ctorParameters', () => {
+    it('doesn\'t process files without decorators/ctorParameters/outside Angular', () => {
       const input = oneLine`
         var Clazz = (function () { function Clazz() { } return Clazz; }());
         ${staticProperty}
diff --git a/packages/angular_devkit/build_optimizer/src/build-optimizer/webpack-loader.ts b/packages/angular_devkit/build_optimizer/src/build-optimizer/webpack-loader.ts
index 0d02b8551d..56a0738d75 100644
--- a/packages/angular_devkit/build_optimizer/src/build-optimizer/webpack-loader.ts
+++ b/packages/angular_devkit/build_optimizer/src/build-optimizer/webpack-loader.ts
@@ -21,7 +21,11 @@ export default function buildOptimizerLoader
   this.cacheable();
   const options: BuildOptimizerLoaderOptions = loaderUtils.getOptions(this) || {};
 
-  const boOutput = buildOptimizer({ content, emitSourceMap: options.sourceMap });
+  const boOutput = buildOptimizer({
+    content,
+    inputFilePath: this.resourcePath,
+    emitSourceMap: options.sourceMap,
+  });
   const intermediateSourceMap = boOutput.sourceMap;
   let newContent = boOutput.content;