Make file system watcher compile on Mac OS 106, and add a runtime-call to test if the system suppor…

tests/standalone/io/file_system_watcher_test.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:async";
 import "dart:io";
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
@@ skipping 219 matching lines @@
 
   new Timer(const Duration(milliseconds: 300), () {
     sub.cancel();
     asyncEnd();
     dir.deleteSync(recursive: true);
   });
 }
 
 
 void main() {
+  if (!FileSystemEntity.isWatchSupported) return;
   testWatchCreateFile();
   testWatchModifyFile();
   testWatchMoveFile();
   testWatchDeleteFile();
   testWatchOnlyModifyFile();
   testMultipleEvents();
   testWatchNonRecursive();
 }