Make FileInputStream and FileOutputStream actually asynchronous.

runtime/bin/timer_impl.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// 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.
 
 class _Timer implements Timer {
   // Set jitter to wake up timer events that would happen in _TIMER_JITTER ms.
   static final int _TIMER_JITTER = 0;
 
   // Disables the timer.
   static final int _NO_TIMER = -1;
 
@@ skipping 123 matching lines @@
           break;
         }
       }
 
       // Trigger all of the pending timers. New timers added as part of the
       // callbacks will be enqueued now and notified in the next spin at the
       // earliest.
       _handling_callbacks = true;
       try {
         for (var timer in pending_timers) {
-          timer._callback(timer);
-          if (timer._repeating) {
-            timer._advanceWakeupTime();
-            timer._addTimerToList();
+          // One of the timers in the pending_timers list can cancel
+          // one of the later timers which will set the callback to
+          // null.
+          if (timer._callback != null) {
+            timer._callback(timer);
+            if (timer._repeating) {
+              timer._advanceWakeupTime();
+              timer._addTimerToList();
+            }
           }
         }
       } finally {
         _handling_callbacks = false;
       }
       _notifyEventHandler();
     }
 
     if(_receivePort === null) {
       _receivePort = new ReceivePort();
@@ skipping 14 matching lines @@
 
   static ReceivePort _receivePort;
   static bool _handling_callbacks = false;
 
   var _callback;
   int _milliSeconds;
   int _wakeupTime;
   bool _repeating;
 }