Fix hints reported by analyzer.

pkg/compiler/lib/src/common/work.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.common.work;
 
 import '../common.dart';
-import '../compiler.dart' show Compiler;
 import '../elements/elements.dart' show AstElement;
-import '../enqueue.dart' show Enqueuer;
 import '../universe/world_impact.dart' show WorldImpact;
 
 abstract class WorkItem {
   /// Element on which the work will be done.
   final AstElement element;
 
   WorkItem(this.element) {
     assert(invariant(element, element.isDeclaration));
   }
 
   WorldImpact run();
 }