Add dummy dart:html library for documentation.

lib/html/doc/interface/FileSystemCallback.dartdoc

Index: lib/html/doc/interface/FileSystemCallback.dartdoc
diff --git a/lib/html/doc/interface/FileSystemCallback.dartdoc b/lib/html/doc/interface/FileSystemCallback.dartdoc
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index 0000000000000000000000000000000000000000..1b86ca9d584777deaab23784d03bb8aa6eab5abb
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+++ b/lib/html/doc/interface/FileSystemCallback.dartdoc
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+// 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.
+
+// WARNING:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+typedef bool FileSystemCallback(DOMFileSystem fileSystem);