Reduce timer stack overhead by getting immediate callback and run it, instead of calling through fu…

runtime/lib/timer_impl.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.
 
 // Timer heap implemented as a array-based binary heap[0].
 // This allows for O(1) `first`, O(log(n)) `remove`/`removeFirst` and O(log(n))
 // `add`.
 //
 // To ensure the timers are ordered by insertion time, the _Timer class has a
 // `_id` field set when added to the heap.
@@ skipping 378 matching lines @@
             // Mark timer as inactive.
             timer._callback = null;
           }
           callback(timer);
           // Re-insert repeating timer if not canceled.
           if (timer._repeating && (timer._callback != null)) {
             timer._advanceWakeupTime();
             timer._addTimerToHeap();
           }
           // Execute pending micro tasks.
-          _runPendingImmediateCallback();
+          var immediateCallback = _removePendingImmediateCallback();
+          if (immediateCallback != null) {
+            immediateCallback();
+          }
         }
       }
     } finally {
       _handlingCallbacks = false;
       _notifyEventHandler();
     }
   }
 
   // Creates a receive port and registers an empty handler on that port. Just
   // the triggering of the event loop will ensure that timers are executed.
@@ skipping 24 matching lines @@
     if (repeating) {
       return new _Timer.periodic(milliSeconds, callback);
     }
     return new _Timer(milliSeconds, callback);
   }
 }
 
 _setupHooks() {
   VMLibraryHooks.timerFactory = _Timer._factory;
 }