With this change, they're run via instance methods on an object
obtained through the context. This will allow us to substitute
that object in tests with replay/record versions to allow us to
mock out the os-layer in tests.
* Remove the workaround that pinned args to v0.13.6
This reverts most of the changes in commit 6331b6c8b5
* 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
This prevents multiple simultaneous runs of the analyzer from stomping
over each other (e.g. multiple runs of 'update-packages'). Certain
long-lived commands (like analyze, run, logs) are exempted once they've
done enough work to be safe from most stomping action.
This still doesn't make us entirely safe from craziness, e.g. if you're
half way through an 'update-packages' run and you call 'git pull', who
knows what state you'll end up in. But there's only so much one can do.
Fixes https://github.com/flutter/flutter/issues/2762
