Remove explicit interface versions of dartdoc files.

lib/html/doc/interface/MediaKeyEvent.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 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;
-}