Decouple WorkItem from Compiler

pkg/compiler/lib/src/enqueue.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.enqueue;
 
 import 'dart:collection' show Queue;
 
 import 'cache_strategy.dart';
 import 'common/backend_api.dart' show Backend;
@@ skipping 527 matching lines @@
       _universe.closuresWithFreeTypeVariables.add(element);
     }
     applyImpact(backend.registerBoundClosure());
     _universe.closurizedMembers.add(element);
   }
 
   void forEach(void f(WorkItem work)) {
     do {
       while (queue.isNotEmpty) {
         // TODO(johnniwinther): Find an optimal process order.
-        strategy.processWorkItem(f, queue.removeLast());
+        WorkItem work = queue.removeLast();
+        if (!isProcessed(work.element)) {
+          strategy.processWorkItem(f, work);
+          registerProcessedElement(work.element);
+        }
       }
       List recents = recentClasses.toList(growable: false);
       recentClasses.clear();
       if (!onQueueEmpty(recents)) recentClasses.addAll(recents);
     } while (queue.isNotEmpty || recentClasses.isNotEmpty);
   }
 
   void logSummary(log(message)) {
     log('Resolved ${processedElements.length} elements.');
     nativeEnqueuer.logSummary(log);
@@ skipping 244 matching lines @@
 }
 
 typedef void _DeferredActionFunction();
 
 class _DeferredAction {
   final Element element;
   final _DeferredActionFunction action;
 
   _DeferredAction(this.element, this.action);
 }