Move all tests using dart:io to a separate directory

tests/standalone/src/io/TimerCancelTest.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.
 
 #import("dart:io");
 
 void testSimpleTimer() {
   Timer cancelTimer;
   int repeatTimer;
 
@@ skipping 22 matching lines @@
 void testCancelTimerWithSameTime() {
   var t2;
   var t1 = new Timer((t) => t2.cancel(), 0);
   t2 = new Timer((t) => t1.cancel(), 0);
 }
 
 main() {
   testSimpleTimer();
   testCancelTimerWithSameTime();
 }