Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/js_backend/backend_helpers.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.js_backend.helpers;
 
-import '../common.dart';
 import '../common/names.dart' show Identifiers, Uris;
 import '../common_elements.dart' show CommonElements, ElementEnvironment;
 import '../elements/elements.dart' show PublicName;
 import '../elements/entities.dart';
 import '../library_loader.dart' show LoadedLibraries;
 import '../universe/call_structure.dart' show CallStructure;
 import '../universe/selector.dart' show Selector;
 import 'constant_system_javascript.dart';
 import 'js_backend.dart';
 
@@ skipping 760 matching lines @@
   /// The class for patch annotations defined in dart:_js_helper.
   ClassEntity get patchAnnotationClass =>
       _patchAnnotationClass ??= _findHelperClass('_Patch');
 
   ClassEntity _nativeAnnotationClass;
 
   /// The class for native annotations defined in dart:_js_helper.
   ClassEntity get nativeAnnotationClass =>
       _nativeAnnotationClass ??= _findHelperClass('Native');
 }