This relands https://github.com/flutter/flutter/pull/35297
The followings have been done to fix the broken tests:
1. Add `didSendFirstFrameRasterizedEvent` extension and its tests
2. Wait for `didSendFirstFrameRasterizedEvent` instead of
`didSendFirstFrameEvent` during start up tests
3. Mark missed (probably newly added) start up tests as flaky
* Fix TODO syntax.
* Clarify messages for some timeouts, to aid debugging.
* Increase some other timeouts that were a needlessly short, to reduce sources of flakes.
* Remove some more timeouts that were mostly redundant, to remove complexity.
* Minor style cleanup.
* Remove some dangerous traps (specifically, hide the explicit start/end times in TimedEvent since they shouldn't matter).
* Upgrade everything except matcher.
* Roll matcher (and test)
* Adjust tests that depend on flutter:test directly to depend on a shim
* Require use of package:test shim and remove other references to package:test
