Update Timer API to take the callback as the last parameter.

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;
 
   void timeoutHandlerUnreachable(Timer timer) {
     Expect.equals(true, false);
   }
 
   void timeoutHandler(Timer timer) {
     cancelTimer.cancel();
   }
 
   void timeoutHandlerRepeat(Timer timer) {
     repeatTimer++;
     timer.cancel();
     Expect.equals(true, repeatTimer == 1);
   }
 
-  cancelTimer = new Timer(timeoutHandlerUnreachable, 1000);
+  cancelTimer = new Timer(1000, timeoutHandlerUnreachable);
   cancelTimer.cancel();
-  new Timer(timeoutHandler, 1000);
-  cancelTimer = new Timer(timeoutHandlerUnreachable, 2000);
+  new Timer(1000, timeoutHandler);
+  cancelTimer = new Timer(2000, timeoutHandlerUnreachable);
   repeatTimer = 0;
-  new Timer.repeating(timeoutHandlerRepeat, 1500);
+  new Timer.repeating(1500, timeoutHandlerRepeat);
 }
 
 void testCancelTimerWithSameTime() {
   var t2;
-  var t1 = new Timer((t) => t2.cancel(), 0);
-  t2 = new Timer((t) => t1.cancel(), 0);
+  var t1 = new Timer(0, (t) => t2.cancel());
+  t2 = new Timer(0, (t) => t1.cancel());
 }
 
 main() {
   testSimpleTimer();
   testCancelTimerWithSameTime();
 }