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

Closes #8316

modules/angular2/test/web_workers/debug_tools/web_socket_message_bus_spec.dart

@@ -9,8 +9,7 @@ import "package:angular2/testing_internal.dart"
         expect,
         beforeEach,
         beforeEachProviders,
-        SpyObject,
-        proxy;
+        SpyObject;
 import "package:angular2/src/web_workers/debug_tools/web_socket_message_bus.dart";
 import "dart:html" show WebSocket, MessageEvent;
 import "./message_bus_common.dart";

modules/angular2/test/web_workers/shared/message_bus_spec.ts

@@ -6,8 +6,7 @@ import {
   expect,
   beforeEach,
   beforeEachProviders,
-  SpyObject,
-  proxy
+  SpyObject
 } from 'angular2/testing_internal';
 import {ObservableWrapper, TimerWrapper} from 'angular2/src/facade/async';
 import {MessageBus} from 'angular2/src/web_workers/shared/message_bus';

modules/angular2/test/web_workers/shared/service_message_broker_spec.ts

@@ -8,7 +8,6 @@ import {
   beforeEach,
   beforeEachProviders,
   SpyObject,
-  proxy,
   browserDetection
 } from 'angular2/testing_internal';
 import {createPairedMessageBuses} from '../shared/web_worker_test_util';