* Added '--check-for-remote-artifacts' option for Flutter Doctor.
This option takes a Flutter engine revision and issues HEAD requests to
determine whether or not artifacts for the provided engine revision are
available from cloud storage. This functionality is needed for the Dart
SDK autoroller to avoid creating a PR to roll the engine into the
framework while artifacts haven't finished building, which would cause
Cirrus tests to fail.
* 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
