This ensures that accidental usages of dart:io's
file API don't creep in over time.

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

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

* enhance flutter upgrade to find project root
fixes https://github.com/flutter/flutter/issues/2990

* address comments

* restore original formatting

* fix lints

* making flutter run work with gradle

* locate android studio

* add test for settings

* review comments

* more terse flutter upgrade

* fix an issue when updating and the engine update code has changed

* call flutter precache; add a --no-color hidden option

* fix a lint related to getters/setters