fix(@angular-devkit/build-optimizer): don't search for pure top-level functions inside arrow functions

Partially address angular/devkit#816.

Author: filipesilva

Diff for: packages/angular_devkit/build_optimizer/src/transforms/prefix-functions.ts

@@ -53,10 +53,12 @@ export function findTopLevelFunctions(parentNode: ts.Node): Set<ts.Node> {
   const topLevelFunctions = new Set<ts.Node>();
 
   function cb(node: ts.Node) {
-    // Stop recursing into this branch if it's a function expression or declaration, or a class.
+    // Stop recursing into this branch if it's a function expression or declaration, a class, or
+    // a arrow function (lambda).
     if (ts.isFunctionDeclaration(node)
       || ts.isFunctionExpression(node)
       || ts.isClassDeclaration(node)
+      || ts.isArrowFunction(node)
     ) {
       return;
     }

Diff for: packages/angular_devkit/build_optimizer/src/transforms/prefix-functions_spec.ts

@@ -143,5 +143,24 @@ describe('prefix-functions', () => {
 
       expect(tags.oneLine`${transform(input)}`).toEqual(tags.oneLine`${output}`);
     });
+
+    it('doesn\'t add comment when inside arrow function', () => {
+      const input = tags.stripIndent`
+        export const subscribeToArray = (array) => (subscriber) => {
+            for (let i = 0, len = array.length; i < len && !subscriber.closed; i++) {
+                subscriber.next(array[i]);
+            }
+            if (!subscriber.closed) {
+                subscriber.complete();
+            }
+        };
+      `;
+      const output = tags.stripIndent`
+        ${emptyImportsComment}
+        ${input}
+      `;
+
+      expect(tags.oneLine`${transform(input)}`).toEqual(tags.oneLine`${output}`);
+    });
   });
 });