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

pkg/compiler/lib/src/native/enqueue.dart

 // Copyright (c) 2014, 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.
 
-import '../common.dart';
 import '../common/backend_api.dart';
 import '../common_elements.dart' show CommonElements, ElementEnvironment;
 import '../elements/elements.dart';
 import '../elements/entities.dart';
 import '../elements/resolution_types.dart';
 import '../elements/types.dart';
 import '../js_backend/backend_helpers.dart' show BackendHelpers;
 import '../js_backend/backend_usage.dart' show BackendUsageBuilder;
 import '../js_backend/native_data.dart' show NativeData;
 import '../js_emitter/js_emitter.dart' show CodeEmitterTask, NativeEmitter;
@@ skipping 296 matching lines @@
         emitter.directSubtypes.putIfAbsent(superclass, () => <ClassEntity>[]);
     directSubtypes.add(cls);
   }
 
   void logSummary(void log(String message)) {
     super.logSummary(log);
     log('Compiled ${_registeredClasses.length} native classes, '
         '${_unusedClasses.length} native classes omitted.');
   }
 }