Remove stale files from lib/html/src.

lib/html/src/CustomEvent.dart

Index: lib/html/src/CustomEvent.dart
diff --git a/lib/html/src/CustomEvent.dart b/lib/html/src/CustomEvent.dart
deleted file mode 100644
index 2fa0615447e0519a6dcb7cdc9f0c958e8b7f1e5a..0000000000000000000000000000000000000000
--- a/lib/html/src/CustomEvent.dart
+++ /dev/null
@@ -1,10 +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 CustomEvent extends Event default CustomEventWrappingImplementation {
-
-  CustomEvent(String type, [bool canBubble, bool cancelable, Object detail]);
-
-  String get detail();
-}