Paying off technical debt...

This updates AppContext per the recommendations in #15352

Fixes #15352

* executable.dart#main() depends on runner.dart#run()
* Refactor code such that non-commands don't depend on commands.

No code was actually changed in this PR - code was merely moved from
point A to point B.

* address unused element issues and new warnings

* add a missing return

* enable crash reporting in flutter_tools

* fix analytics text; use relative paths

* fix test

`all.dart` is no longer needed. Furthermore, it causes tests to
be skipped, or to silently fail to run anything.

Fixes #7941

* prefer const constructors

* fix double throw

* add crash reporting without enabling it

* do not drop futures on the floor

* return exitCode from executable run

* debug travis

* remove unnecessary todo

* rename local fs getter