Ignore unused classes when constructing set for deferred loading.

sdk/lib/_internal/compiler/implementation/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;
 
 class EnqueueTask extends CompilerTask {
   final ResolutionEnqueuer resolution;
   final CodegenEnqueuer codegen;
 
@@ skipping 554 matching lines @@
                      ItemCompilationContext itemCompilationContextCreator())
       : super('resolution enqueuer', compiler, itemCompilationContextCreator),
         resolvedElements = new LinkedHashMap<Element, TreeElements>(),
         queue = new Queue<ResolutionWorkItem>(),
         postQueue = new Queue<PostProcessTask>();
 
   bool get isResolutionQueue => true;
 
   bool isProcessed(Element member) => resolvedElements.containsKey(member);
 
+  /// Returns [:true:] if [element] has actually been used.
+  bool isLive(Element element) {
+    if (seenClasses.contains(element)) return true;
+    if (getCachedElements(element) != null) return true;
+    return false;
+  }
+
   TreeElements getCachedElements(Element element) {
     // TODO(ngeoffray): Get rid of this check.
     if (element.enclosingElement.isClosure()) {
       closureMapping.ClosureClassElement cls = element.enclosingElement;
       element = cls.methodElement;
     } else if (element.isGenerativeConstructorBody()) {
       ConstructorBodyElement body = element;
       element = body.constructor;
     }
     Element owner = element.getOutermostEnclosingMemberOrTopLevel();
@@ skipping 141 matching lines @@
     while(!queue.isEmpty) {
       // TODO(johnniwinther): Find an optimal process order for codegen.
       f(queue.removeLast());
     }
   }
 
   void _logSpecificSummary(log(message)) {
     log('Compiled ${generatedCode.length} methods.');
   }
 }