Use the event-handler for timers.

tests/debugger/async_expected.txt

 Starting session. Type 'help' for a list of commands.
 
 > b breakHere
 breakpoint set: id: '0' method: 'breakHere' bytecode index: '0'
 > r
 Stack trace:
-> 0: breakHere                  @void breakHere(){}
+> 0: breakHere                          @void breakHere(){}
 > t internal
 Stack trace:
-> 0: breakHere                  @void breakHere(){}
-  1: call                       
-  2: Future.call                @computation()
-  3: _handleEvents              @event()
-  4: call                       
-  5: runToEnd                   @entry()
-  6: Fiber.call                 @fletch.runToEnd(entry)
-  7: Coroutine._coroutineStart  @entry(argument)
+> 0: breakHere                          @void breakHere(){}
+  1: call                               
+  2: Future.call                        @computation()
+  3: Object._noSuchMethodTrampoline     
+  4: _FletchTimer.call                  @_callback()
+  5: runToEnd                           @entry()
+  6: Fiber.call                         @fletch.runToEnd(entry)
+  7: Coroutine._coroutineStart          @entry(argument)
 > c
 ### process terminated