Finish strong mode cleaning of dart2js.

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/tasks.dart' show CompilerTask;
 import '../common/work.dart' show WorkItem;
@@ skipping 211 matching lines @@
       while (_queue.isNotEmpty) {
         // TODO(johnniwinther): Find an optimal process order.
         WorkItem work = _queue.removeLast();
         if (!_processedEntities.contains(work.element)) {
           strategy.processWorkItem(f, work);
           // TODO(johnniwinther): Register the processed element here. This
           // is currently a side-effect of calling `work.run`.
           _processedEntities.add(work.element);
         }
       }
-      List recents = _recentClasses.toList(growable: false);
+      List<ClassEntity> recents = _recentClasses.toList(growable: false);
       _recentClasses.clear();
       _recentConstants = false;
       if (!_onQueueEmpty(recents)) _recentClasses.addAll(recents);
     } while (
         _queue.isNotEmpty || _recentClasses.isNotEmpty || _recentConstants);
   }
 
   /// [_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 13 matching lines @@
   String toString() => 'Enqueuer($name)';
 
   ImpactUseCase get impactUse => IMPACT_USE;
 
   @override
   Iterable<MemberEntity> get processedEntities => _processedEntities;
 
   @override
   Iterable<ClassEntity> get processedClasses => _worldBuilder.processedClasses;
 }