Add dummy dart:html library for documentation.

lib/html/doc/interface/FileWriterSync.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 FileWriterSync
+interface FileWriterSync {
+
+  /** @domName FileWriterSync.length */
+  final int length;
+
+  /** @domName FileWriterSync.position */
+  final int position;
+
+  /** @domName FileWriterSync.seek */
+  void seek(int position);
+
+  /** @domName FileWriterSync.truncate */
+  void truncate(int size);
+
+  /** @domName FileWriterSync.write */
+  void write(Blob data);
+}