Decouple SSA builder from codegen-work-item

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 'common.dart';
@@ skipping 757 matching lines @@
   String toString() => 'Enqueuer($name)';
 
   void forgetElement(Element element) {
     universe.forgetElement(element, compiler);
     _processedClasses.remove(element);
   }
 }
 
 /// [Enqueuer] which is specific to resolution.
 class ResolutionEnqueuer extends Enqueuer {
-  /**
-   * Map from declaration elements to the [TreeElements] object holding the
-   * resolution mapping for the element implementation.
-   *
-   * Invariant: Key elements are declaration elements.
-   */
+  /// All declaration elements that have been processed by the resolver.
   final Set<AstElement> processedElements;
 
   final Queue<ResolutionWorkItem> queue;
 
   /**
    * A deferred task queue for the resolution phase which is processed
    * when the resolution queue has been emptied.
    */
   final Queue<DeferredTask> deferredTaskQueue;
 
@@ skipping 289 matching lines @@
   @override
   void processStaticUse(Enqueuer enqueuer, Element element) {
     enqueuer.registerStaticUseInternal(element);
   }
 
   @override
   void processSelector(Enqueuer enqueuer, UniverseSelector selector) {
     enqueuer.handleUnseenSelectorInternal(selector);
   }
 }