* Remove the workaround that pinned args to v0.13.6
This reverts most of the changes in commit 6331b6c8
* 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

Implement flutter format, fixes https://github.com/flutter/flutter/issues/5338

* implement flutter packages get/upgrade command
* alias flutter pub
* refactor findProjectRoot for use by multiple commands

fixes https://github.com/flutter/flutter/issues/4994