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

runtime/lib/isolate_patch.dart

Index: runtime/lib/isolate_patch.dart
diff --git a/runtime/lib/isolate_patch.dart b/runtime/lib/isolate_patch.dart
index 9a7443806ff035bead58546c85b6b285b12803de..026c6f6f518c85d8d44d318cd42850d873224d1c 100644
--- a/runtime/lib/isolate_patch.dart
+++ b/runtime/lib/isolate_patch.dart
@@ -97,6 +97,12 @@ void _runPendingImmediateCallback() {
   }
 }
 
+ImmediateCallback _removePendingImmediateCallback() {
+  var callback = _pendingImmediateCallback;
+  _pendingImmediateCallback = null;
+  return callback;
+}
+
 /// The embedder can execute this function to get hold of
 /// [_isolateScheduleImmediate] above.
 Function _getIsolateScheduleImmediateClosure() {