Let Backend determine support for deferred loading.

pkg/compiler/lib/src/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;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 2651 matching lines @@
       enqueuer.registerInstantiatedType(
           clsASyncStarController.rawType, registry);
       enqueue(enqueuer, getAsyncStarHelper(), registry);
       enqueue(enqueuer, getStreamOfController(), registry);
       enqueue(enqueuer, getYieldSingle(), registry);
       enqueue(enqueuer, getYieldStar(), registry);
       enqueue(enqueuer, getASyncStarControllerConstructor(), registry);
       enqueue(enqueuer, getStreamIteratorConstructor(), registry);
     }
   }
+
+  @override
+  bool registerDeferredLoading(Spannable node, Registry registry) {
+    registerCheckDeferredIsLoaded(registry);
+    return true;
+  }
 }
 
 /// Handling of special annotations for tests.
 class Annotations {
   static final Uri PACKAGE_EXPECT =
       new Uri(scheme: 'package', path: 'expect/expect.dart');
 
   final Compiler compiler;
 
   ClassElement expectNoInlineClass;
@@ skipping 269 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }