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

pkg/compiler/lib/src/js_backend/type_variable_handler.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.
 
-import '../common.dart';
 import '../common_elements.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 import '../elements/resolution_types.dart';
 import '../elements/elements.dart';
 import '../js/js.dart' as jsAst;
 import '../js_emitter/js_emitter.dart'
     show CodeEmitterTask, MetadataCollector, Placeholder;
 import '../universe/call_structure.dart' show CallStructure;
 import '../universe/use.dart' show ConstantUse;
@@ skipping 155 matching lines @@
   }
 
   List<jsAst.Expression> typeVariablesOf(ClassElement classElement) {
     List<jsAst.Expression> result = _typeVariables[classElement];
     if (result == null) {
       result = const <jsAst.Expression>[];
     }
     return result;
   }
 }