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

pkg/compiler/lib/src/common/backend_api.dart

 // Copyright (c) 2012, 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.backend_api;
 
-import '../common.dart';
-import '../common/codegen.dart' show CodegenImpact;
 import '../common/resolution.dart' show ResolutionImpact;
 import '../constants/expressions.dart' show ConstantExpression;
 import '../elements/types.dart';
 import '../elements/resolution_types.dart'
     show ResolutionDartType, ResolutionInterfaceType;
 import '../elements/entities.dart';
 import '../enqueue.dart' show ResolutionEnqueuer;
 import '../serialization/serialization.dart'
     show DeserializerPlugin, SerializerPlugin;
 import '../tree/tree.dart' show Node;
@@ skipping 162 matching lines @@
   InterfaceType getConstantMapTypeFor(InterfaceType sourceType,
       {bool hasProtoKey: false, bool onlyStringKeys: false});
 
   /// Returns the type of the constant symbol implementation class.
   InterfaceType get symbolType;
 
   /// Returns the field of the constant symbol implementation class that holds
   /// its internal name.
   FieldEntity get symbolField;
 }