Add support for file locking

sdk/lib/_internal/compiler/js_lib/io_patch.dart

 // Copyright (c) 2013, 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.
 
 import 'dart:_js_helper' show patch;
 
 @patch
 class _Directory {
   @patch
   static _current() {
@@ skipping 166 matching lines @@
     throw new UnsupportedError("RandomAccessFile._truncate");
   }
   @patch
   static _length(int id) {
     throw new UnsupportedError("RandomAccessFile._length");
   }
   @patch
   static _flush(int id) {
     throw new UnsupportedError("RandomAccessFile._flush");
   }
+  @patch
+  static _lock(int id, int lock, int start, int end) {
+    throw new UnsupportedError("RandomAccessFile._lock");
+  }
 }
 
 @patch
 class _IOCrypto {
   @patch
   static Uint8List getRandomBytes(int count) {
     throw new UnsupportedError("_IOCrypto.getRandomBytes");
   }
 }
 
@@ skipping 320 matching lines @@
   }
 }
 
 @patch
 class _IOService {
   @patch
   static Future dispatch(int request, List data) {
     throw new UnsupportedError("_IOService.dispatch");
   }
 }