1. Add matchers for the `ProcessExit` exception class
2. Add ability to control the setup of the `AppContext` we use in
`testUsingContext()`
3. Clean up the code that figures out the location of `Cache.flutterRoot`
such that it works with `pub run test`. It previously only worked
when the tests were invoked with standalone `dart`
`#3` above will also help unblock #7941
- [x] Introduce DependencyChecker which can determine if any dependencies have been modified.
- [x] Move the DartDependencyBuilder into a separate file.
- [x] Add unit tests for DartDependencyBuilder.
- [x] Add unit tets for DependencyChecker
Part of #7014
* Remove the workaround that pinned args to v0.13.6
This reverts most of the changes in commit 6331b6c8b5d964ec0dbf2cd9bb84c60c650a0878
* throw exception if exit code is not an integer
* rework command infrastructure to throw ToolExit when non-zero exitCode
* convert commands to return Future<Null>
* cleanup remaining commands to use throwToolExit for non-zero exit code
* remove isUnusual exception message
* add type annotations for updated args package
