Add dummy dart:html library for documentation.

lib/html/doc/interface/DirectoryEntry.dartdoc

+// 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:
+//   This file contains documentation that is merged into the real source.
+//   Do not make code changes here.
+
+/// @domName DirectoryEntry
+interface DirectoryEntry extends Entry {
+
+  /** @domName DirectoryEntry.createReader */
+  DirectoryReader createReader();
+
+  /** @domName DirectoryEntry.getDirectory */
+  void getDirectory(String path, [Object flags, EntryCallback successCallback, ErrorCallback errorCallback]);
+
+  /** @domName DirectoryEntry.getFile */
+  void getFile(String path, [Object flags, EntryCallback successCallback, ErrorCallback errorCallback]);
+
+  /** @domName DirectoryEntry.removeRecursively */
+  void removeRecursively(VoidCallback successCallback, [ErrorCallback errorCallback]);
+}