* Add a "build" phase to EnginePhase for completeness.
* Ignore events from the device during test execution.
* More dartdocs
* Slightly more helpful messages about Timers in verifyInvariants.
* Add widgetList, elementList, stateList, renderObjectList.
* Send test events asynchronously for consistency with other APIs.
* Fix a test that was depending on test events being synchronous (or
  rather, scheduled in a microtask that came before the microtask for
  the completer of the future that the tap() function returned).