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

pkg/compiler/lib/src/common_elements.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.
 
 // TODO(sigmund): rename and move to common/elements.dart
 library dart2js.type_system;
 
-import 'common.dart';
 import 'common/names.dart' show Uris;
 import 'elements/types.dart';
 import 'elements/entities.dart';
 
 /// The common elements and types in Dart.
 abstract class CommonElements {
   /// The `Object` class defined in 'dart:core'.
   ClassEntity get objectClass;
 
   /// The `bool` class defined in 'dart:core'.
@@ skipping 536 matching lines @@
   }
 
   @override
   InterfaceType streamType([DartType elementType]) {
     if (elementType == null) {
       return getRawType(streamClass);
     }
     return createInterfaceType(streamClass, [elementType]);
   }
 }