Implement MirrorUsed.targets for libraries

dart/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 293 matching lines @@
       registerNewSelector(selector, universe.invokedSetters);
     });
   }
 
   /// Called when [:const Symbol(name):] is seen.
   void registerConstSymbol(String name, TreeElements elements) {
     compiler.backend.registerConstSymbol(name, elements);
   }
 
   void pretendElementWasUsed(Element element, TreeElements elements) {
+    if (!compiler.backend.isNeededForReflection(element)) return;
     if (Elements.isUnresolved(element)) {
       // Ignore.
     } else if (element.isSynthesized
                && element.getLibrary().isPlatformLibrary) {
       // TODO(ahe): Work-around for http://dartbug.com/11205.
     } else if (element.isConstructor()) {
       ClassElement cls = element.declaration.getEnclosingClass();
       registerInstantiatedType(cls.rawType, elements);
       registerStaticUse(element.declaration);
     } else if (element.isMixinApplication) {
@@ skipping 399 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.');
   }
 }