Move dart2js from sdk/lib/_internal/compiler to pkg/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.
 
 part of dart2js;
 
 typedef ItemCompilationContext ItemCompilationContextCreator();
 
 class EnqueueTask extends CompilerTask {
   final ResolutionEnqueuer resolution;
@@ skipping 885 matching lines @@
   void processWorkItem(void f(WorkItem work), WorkItem work) {
     f(work);
   }
 }
 
 void removeFromSet(Map<String, Set<Element>> map, Element element) {
   Set<Element> set = map[element.name];
   if (set == null) return;
   set.remove(element);
 }