Add dummy dart:html library for documentation.

lib/html/doc/interface/MediaKeyEvent.dart

Index: lib/html/doc/interface/MediaKeyEvent.dart
diff --git a/lib/html/doc/interface/MediaKeyEvent.dart b/lib/html/doc/interface/MediaKeyEvent.dart
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index 0000000000000000000000000000000000000000..8a64e399f8251eae2a1f402e56b9bd83125369f8
--- /dev/null
+++ b/lib/html/doc/interface/MediaKeyEvent.dart
@@ -0,0 +1,30 @@
+// 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 MediaKeyEvent
+interface MediaKeyEvent extends Event {
+
+  /** @domName MediaKeyEvent.defaultURL */
+  final String defaultURL;
+
+  /** @domName MediaKeyEvent.errorCode */
+  final MediaKeyError errorCode;
+
+  /** @domName MediaKeyEvent.initData */
+  final Uint8Array initData;
+
+  /** @domName MediaKeyEvent.keySystem */
+  final String keySystem;
+
+  /** @domName MediaKeyEvent.message */
+  final Uint8Array message;
+
+  /** @domName MediaKeyEvent.sessionId */
+  final String sessionId;
+
+  /** @domName MediaKeyEvent.systemCode */
+  final int systemCode;
+}