Use entities in class_stub_generator.

pkg/compiler/lib/src/js_emitter/js_emitter.dart

 // Copyright (c) 2013, 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_emitter;
 
-import '../elements/elements.dart'
-    show
-        ClassElement,
-        MixinApplicationElement;
+import '../elements/elements.dart' show ClassElement, MixinApplicationElement;
 
 export 'class_stub_generator.dart';
 export 'code_emitter_task.dart';
 export 'interceptor_stub_generator.dart';
 export 'main_call_stub_generator.dart';
 export 'metadata_collector.dart';
 export 'native_emitter.dart';
 export 'native_generator.dart';
 export 'parameter_stub_generator.dart';
 export 'runtime_type_generator.dart';
 export 'type_test_registry.dart';
 
 part 'helpers.dart';