dart2js: Fix hints in code base.

pkg/compiler/lib/src/deferred_load.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.
 
 library deferred_load;
 
-import 'constants/expressions.dart';
 import 'constants/values.dart' show
     ConstantValue,
     ConstructedConstantValue,
     DeferredConstantValue,
     StringConstantValue;
 
 import 'dart2jslib.dart' show
     Backend,
     Compiler,
     CompilerTask,
     invariant,
     MessageKind;
 
-import 'dart_backend/dart_backend.dart' show
-    DartBackend;
-
 import 'js_backend/js_backend.dart' show
     JavaScriptBackend;
 
 import 'elements/elements.dart' show
     AccessorElement,
     AstElement,
     ClassElement,
     Element,
     ElementKind,
     Elements,
@@ skipping 854 matching lines @@
         _importingLibrary = importingLibrary;
 
   String get importingLibraryName {
     String libraryName = _importingLibrary.getLibraryName();
     return libraryName == ""
       ? "<unnamed>"
       : libraryName;
   }
 
 }