Move all tests using dart:io to a separate directory

tests/standalone/src/io/FileOutputStreamTest.dart

 // 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.
 // Testing file input stream, VM-only, standalone test.
 
 #import("dart:io");
 #import("dart:isolate");
 
 void testOpenOutputStreamSync() {
   Directory tempDirectory = new Directory('');
@@ skipping 57 matching lines @@
       };
     });
   });
 }
 
 
 main() {
   testOpenOutputStreamSync();
   testOutputStreamNoPendingWrite();
 }