Decouple WorkItem from Compiler

pkg/compiler/lib/src/js_backend/enqueuer.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.js.enqueue;
 
 import 'dart:collection' show Queue;
 
 import '../common/backend_api.dart' show Backend;
 import '../common/codegen.dart' show CodegenWorkItem;
@@ skipping 96 matching lines @@
     }
 
     if (options.hasIncrementalSupport && !isProcessed(element)) {
       newlyEnqueuedElements.add(element);
     }
 
     if (queueIsClosed) {
       throw new SpannableAssertionFailure(
           element, "Codegen work list is closed. Trying to add $element");
     }
-    queue.add(new CodegenWorkItem(_compiler, element));
+    queue.add(new CodegenWorkItem(backend, element));
     // TODO(sigmund): add other missing dependencies (internals, selectors
     // enqueued after allocations).
     _compiler.dumpInfoTask
         .registerDependency(_compiler.currentElement, element);
   }
 
   void applyImpact(WorldImpact worldImpact, {Element impactSource}) {
     if (worldImpact.isEmpty) return;
     impactStrategy.visitImpact(
         impactSource, worldImpact, impactVisitor, impactUse);
@@ skipping 312 matching lines @@
       applyImpact(backend.registerClosureWithFreeTypeVariables(element,
           forResolution: false));
     }
     applyImpact(backend.registerBoundClosure());
   }
 
   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);
+          // TODO(johnniwinther): Register the processed element here. This
+          // is currently a side-effect of calling `work.run`.
+        }
       }
       List recents = recentClasses.toList(growable: false);
       recentClasses.clear();
       if (!onQueueEmpty(recents)) recentClasses.addAll(recents);
     } while (queue.isNotEmpty || recentClasses.isNotEmpty);
   }
 
   /// [onQueueEmpty] is called whenever the queue is drained. [recentClasses]
   /// contains the set of all classes seen for the first time since
   /// [onQueueEmpty] was called last. A return value of [true] indicates that
@@ skipping 70 matching lines @@
 
   @override
   Iterable<ClassElement> get processedClasses => _processedClasses;
 }
 
 void removeFromSet(Map<String, Set<Element>> map, Element element) {
   Set<Element> set = map[element.name];
   if (set == null) return;
   set.remove(element);
 }