diff --git a/packages/angular_devkit/build_optimizer/src/transforms/prefix-functions.ts b/packages/angular_devkit/build_optimizer/src/transforms/prefix-functions.ts
index d2414bb98970..a692862a87bb 100644
--- a/packages/angular_devkit/build_optimizer/src/transforms/prefix-functions.ts
+++ b/packages/angular_devkit/build_optimizer/src/transforms/prefix-functions.ts
@@ -47,6 +47,7 @@ export function findTopLevelFunctions(parentNode: ts.Node): Set<ts.Node> {
     if (ts.isFunctionDeclaration(node)
       || ts.isFunctionExpression(node)
       || ts.isClassDeclaration(node)
+      || ts.isClassExpression(node)
       || ts.isArrowFunction(node)
       || ts.isMethodDeclaration(node)
     ) {
diff --git a/packages/angular_devkit/build_optimizer/src/transforms/prefix-functions_spec.ts b/packages/angular_devkit/build_optimizer/src/transforms/prefix-functions_spec.ts
index d8b55c65a5d4..10c444afb057 100644
--- a/packages/angular_devkit/build_optimizer/src/transforms/prefix-functions_spec.ts
+++ b/packages/angular_devkit/build_optimizer/src/transforms/prefix-functions_spec.ts
@@ -167,4 +167,23 @@ describe('prefix-functions', () => {
 
     expect(tags.oneLine`${transform(input)}`).toEqual(tags.oneLine`${output}`);
   });
+
+  it('doesn\'t add comment when inside class expression', () => {
+    const input = tags.stripIndent`
+      let Foo = class Foo {
+        constructor() {
+          this.isExpandedChange = new EventEmitter();
+        }
+
+        set isExpanded(value) {
+          this.isExpandedChange.emit(value);
+        }
+      };
+    `;
+    const output = tags.stripIndent`
+      ${input}
+    `;
+
+    expect(tags.oneLine`${transform(input)}`).toEqual(tags.oneLine`${output}`);
+  });
 });