test(@angular-devkit/build-angular): re-enable karma es2015 test

Author: filipesilva

packages/angular_devkit/build_angular/test/karma/works_spec_large.ts

@@ -29,9 +29,7 @@ describe('Karma Builder', () => {
     ).subscribe(undefined, done.fail, done);
   }, 30000);
 
-  // TODO: this test is failing with `TypeError: Assignment to constant variable.` errors.
-  // Need to investigate why. Might be TS 2.7.
-  xit('supports ES2015 target', (done) => {
+  it('supports ES2015 target', (done) => {
     host.replaceInFile('tsconfig.json', '"target": "es5"', '"target": "es2015"');
     runTargetSpec(host, karmaTargetSpec).pipe(
       tap((buildEvent) => expect(buildEvent.success).toBe(true)),