Add dummy dart:html library for documentation.

lib/html/doc/interface/AnimationEvent.dart

Index: lib/html/doc/interface/AnimationEvent.dart
diff --git a/lib/html/doc/interface/AnimationEvent.dart b/lib/html/doc/interface/AnimationEvent.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c7e7b84a75ccc4a700a0761f8abd73eb10d36b70
--- /dev/null
+++ b/lib/html/doc/interface/AnimationEvent.dart
@@ -0,0 +1,15 @@
+// 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.
+
+// WARNING: Do not edit - generated code.
+
+/// @domName WebKitAnimationEvent
+interface AnimationEvent extends Event {
+
+  /** @domName WebKitAnimationEvent.animationName */
+  final String animationName;
+
+  /** @domName WebKitAnimationEvent.elapsedTime */
+  final num elapsedTime;
+}