First pass at asynchronous input loading in dart2js.

sdk/lib/_internal/compiler/implementation/js_backend/backend.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 js_backend;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked;
 
   JavaScriptItemCompilationContext()
@@ skipping 1471 matching lines @@
     if (mustRetainMetadata && isNeededForReflection(element)) {
       for (MetadataAnnotation metadata in element.metadata) {
         metadata.ensureResolved(compiler)
             .value.accept(new ConstantCopier(compiler.constantHandler));
       }
       return true;
     }
     return false;
   }
 
-  void onLibraryLoaded(LibraryElement library, Uri uri) {
+  Future onLibraryLoaded(LibraryElement library, Uri uri) {
     if (uri == Uri.parse('dart:_js_mirrors')) {
       disableTreeShakingMarker =
           library.find(const SourceString('disableTreeShaking'));
       preserveMetadataMarker =
           library.find(const SourceString('preserveMetadata'));
     } else if (uri == Uri.parse('dart:_js_names')) {
       preserveNamesMarker =
           library.find(const SourceString('preserveNames'));
     }
+    return new Future.value();
   }
 
   void registerMetadataInstantiatedType(DartType type, TreeElements elements) {
     if (mustRetainMetadata) {
       compiler.constantHandler.registerInstantiatedType(type, elements);
     } else {
       metadataInstantiatedTypes.add(new Dependency(type, elements));
     }
   }
 
@@ skipping 171 matching lines @@
     copy(constant.values);
     copy(constant.protoValue);
     copy(constant);
   }
 
   void visitConstructed(ConstructedConstant constant) {
     copy(constant.fields);
     copy(constant);
   }
 }