Skeleton API for lazy library loading.

dart/sdk/lib/_internal/compiler/implementation/lib/async_patch.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.
 
 // Patch file for the dart:async library.
 
 import 'dart:_isolate_helper' show TimerImpl;
 
 typedef void _TimerCallback0();
 typedef void _TimerCallback1(Timer timer);
@@ skipping 20 matching lines @@
    * [milliseconds] millisecond until cancelled.
    */
   patch factory Timer.repeating(var duration, void callback(Timer timer)) {
     // TODO(floitsch): remove this check when we remove the deprecated
     // millisecond argument.
     int milliseconds = duration is int ? duration : duration.inMilliseconds;
     if (milliseconds < 0) milliseconds = 0;
     return new TimerImpl.repeating(milliseconds, callback);
   }
 }
+
+final Set<String> _loadedLibraries = new Set<String>();
+
+patch class DeferredLibrary {
+  patch Future<bool> load() {
+    // TODO(ahe): Implement this.
+    Future future =
+        new Future<bool>.immediate(!_loadedLibraries.contains(libraryName));
+    _loadedLibraries.add(libraryName);
+    return future;
+  }
+}