fix(@angular-devkit/build-optimizer): don't add pure comments inside arrow functions

Fixes #13768

packages/angular_devkit/build_optimizer/src/transforms/prefix-functions.ts

@@ -64,6 +64,14 @@ export function findTopLevelFunctions(parentNode: ts.Node): Set<ts.Node> {
       return;
     }
 
+    if ((ts.isFunctionExpression(innerNode) || ts.isArrowFunction(innerNode))
+      && ts.isParenthesizedExpression(node)) {
+        // pure functions can be wrapped in parentizes
+        // we should not add pure comments to this sort of syntax.
+        // example var foo = (() => x)
+      return;
+    }
+
     if (noPureComment) {
       if (ts.isNewExpression(innerNode)) {
         topLevelFunctions.add(node);

packages/angular_devkit/build_optimizer/src/transforms/prefix-functions_spec.ts

@@ -141,4 +141,30 @@ describe('prefix-functions', () => {
       expect(tags.oneLine`${transform(input)}`).toEqual(tags.oneLine`${output}`);
     });
   });
+
+  it('doesn\'t add comment to downlevel arrow function', () => {
+    const input = tags.stripIndent`
+      var populate = (function (props, rawData, entity) {
+          props.forEach(function (prop) { });
+      });
+    `;
+    const output = tags.stripIndent`
+      ${input}
+    `;
+
+    expect(tags.oneLine`${transform(input)}`).toEqual(tags.oneLine`${output}`);
+  });
+
+  it('doesn\'t add comment inside arrow function', () => {
+    const input = tags.stripIndent`
+      const populate = ((props, rawData, entity) => {
+          props.forEach(x => x);
+      });
+    `;
+    const output = tags.stripIndent`
+      ${input}
+    `;
+
+    expect(tags.oneLine`${transform(input)}`).toEqual(tags.oneLine`${output}`);
+  });
 });