Add dummy dart:html library for documentation.

lib/html/doc/interface/DirectoryEntrySync.dartdoc

Index: lib/html/doc/interface/DirectoryEntrySync.dartdoc
diff --git a/lib/html/doc/interface/DirectoryEntrySync.dartdoc b/lib/html/doc/interface/DirectoryEntrySync.dartdoc
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+// 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 DirectoryEntrySync
+interface DirectoryEntrySync extends EntrySync {
+
+  /** @domName DirectoryEntrySync.createReader */
+  DirectoryReaderSync createReader();
+
+  /** @domName DirectoryEntrySync.getDirectory */
+  DirectoryEntrySync getDirectory(String path, Object flags);
+
+  /** @domName DirectoryEntrySync.getFile */
+  FileEntrySync getFile(String path, Object flags);
+
+  /** @domName DirectoryEntrySync.removeRecursively */
+  void removeRecursively();
+}