Use common.dart to re-export commonly used entities.

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 '../diagnostics/invariant.dart' show
-    invariant;
 import '../elements/elements.dart' show
     AstElement;
 import '../enqueue.dart' show
     Enqueuer,
     WorldImpact;
 
 
 /**
  * Contains backend-specific data that is used throughout the compilation of
  * one work item.
  */
 class ItemCompilationContext {
 }
 
 abstract class WorkItem {
   final ItemCompilationContext compilationContext;
   /**
    * Documentation wanted -- johnniwinther
    *
    * Invariant: [element] must be a declaration element.
    */
   final AstElement element;
 
   WorkItem(this.element, this.compilationContext) {
     assert(invariant(element, element.isDeclaration));
   }
 
   WorldImpact run(Compiler compiler, Enqueuer world);
 }