Remove stale files from lib/html/src.

lib/html/src/MutationEventWrappingImplementation.dart

-// Copyright (c) 2011, 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.
-
-class MutationEventWrappingImplementation extends EventWrappingImplementation implements MutationEvent {
-  MutationEventWrappingImplementation._wrap(ptr) : super._wrap(ptr);
-
-  factory MutationEventWrappingImplementation(String type, Node relatedNode,
-      String prevValue, String newValue, String attrName, int attrChange,
-      [bool canBubble = true, bool cancelable = true]) {
-    final e = dom.document.createEvent("MutationEvent");
-    e.initMutationEvent(type, canBubble, cancelable,
-        LevelDom.unwrap(relatedNode), prevValue, newValue, attrName,
-        attrChange);
-    return LevelDom.wrapMutationEvent(e);
-  }
-
-  int get attrChange() => _ptr.attrChange;
-
-  String get attrName() => _ptr.attrName;
-
-  String get newValue() => _ptr.newValue;
-
-  String get prevValue() => _ptr.prevValue;
-
-  Node get relatedNode() => LevelDom.wrapNode(_ptr.relatedNode);
-}