Add dummy dart:html library for documentation.

lib/html/doc/interface/MediaKeyError.dartdoc

+// 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:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName MediaKeyError
+interface MediaKeyError {
+
+  static final int MEDIA_KEYERR_CLIENT = 2;
+
+  static final int MEDIA_KEYERR_DOMAIN = 6;
+
+  static final int MEDIA_KEYERR_HARDWARECHANGE = 5;
+
+  static final int MEDIA_KEYERR_OUTPUT = 4;
+
+  static final int MEDIA_KEYERR_SERVICE = 3;
+
+  static final int MEDIA_KEYERR_UNKNOWN = 1;
+
+  /** @domName MediaKeyError.code */
+  final int code;
+}