Remove stale files from lib/html/src.

lib/html/src/MutationEvent.dart

Index: lib/html/src/MutationEvent.dart
diff --git a/lib/html/src/MutationEvent.dart b/lib/html/src/MutationEvent.dart
deleted file mode 100644
index 42261b19313db0d2fc6b810ed6fe6e07f21e22ef..0000000000000000000000000000000000000000
--- a/lib/html/src/MutationEvent.dart
+++ /dev/null
@@ -1,26 +0,0 @@
-// 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.
-
-interface MutationEvent extends Event default MutationEventWrappingImplementation {
-
-  MutationEvent(String type, Node relatedNode, String prevValue,
-      String newValue, String attrName, int attrChange, [bool canBubble,
-      bool cancelable]);
-
-  static const int ADDITION = 2;
-
-  static const int MODIFICATION = 1;
-
-  static const int REMOVAL = 3;
-
-  int get attrChange();
-
-  String get attrName();
-
-  String get newValue();
-
-  String get prevValue();
-
-  Node get relatedNode();
-}