fix(build): Resolve Dart analyzer issues with the Dart dev channel

Closes #8316

modules/angular2/test/platform/dom/shared_styles_host_spec.ts

@@ -12,8 +12,7 @@ import {
   beforeEachProviders,
   it,
   xit,
-  SpyObject,
-  proxy
+  SpyObject
 } from 'angular2/testing_internal';
 
 import {DOM} from 'angular2/src/platform/dom/dom_adapter';