Add dummy dart:html library for documentation.

lib/html/doc/interface/DOMFileSystem.dart

Index: lib/html/doc/interface/DOMFileSystem.dart
diff --git a/lib/html/doc/interface/DOMFileSystem.dart b/lib/html/doc/interface/DOMFileSystem.dart
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index 0000000000000000000000000000000000000000..22c6a5678e94f70157598fa49d39c619b16f1f63
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+++ b/lib/html/doc/interface/DOMFileSystem.dart
@@ -0,0 +1,15 @@
+// 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: Do not edit - generated code.
+
+/// @domName DOMFileSystem
+interface DOMFileSystem {
+
+  /** @domName DOMFileSystem.name */
+  final String name;
+
+  /** @domName DOMFileSystem.root */
+  final DirectoryEntry root;
+}