Add support for file locking

tests/standalone/io/file_lock_script.dart

+// Copyright (c) 2015, 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.
+//
+// Script used by the file_lock_test.dart test.
+
+import "dart:io";
+
+main(List<String> args) {
+  File file = new File(args[0]);
+  int start = null;
+  int end = null;
+  var  mode = FileLock.EXCLUSIVE;
+  if (args[1] == 'SHARED') {
+    mode = FileLock.SHARED;
+  }
+  if (args[2] != 'null') {
+    start = int.parse(args[2]);
+  }
+  if (args[3] != 'null') {
+    end = int.parse(args[3]);
+  }
+  var raf = file.openSync(mode: WRITE);
+  try {
+    raf.lockSync(mode, start, end);
+    print('LOCK SUCCEEDED');
+  } catch (e) {
+    print('LOCK FAILED');
+  } finally {
+    raf.closeSync();
+  }
+}