Skeleton API for lazy library loading.

dart/sdk/lib/_internal/compiler/implementation/lib/async_patch.dart

Index: dart/sdk/lib/_internal/compiler/implementation/lib/async_patch.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/lib/async_patch.dart b/dart/sdk/lib/_internal/compiler/implementation/lib/async_patch.dart
index 0e0a70d2e4620d53530b8b8eb24c3f0b2809d2d2..64c828963ccac5d3fba22b5aff259489f2a34a43 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/lib/async_patch.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/lib/async_patch.dart
@@ -38,3 +38,15 @@ patch class Timer {
     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;
+  }
+}