* Use a set to remove duplicate channel names

* Add test for git command

In the case of "dev", `git checkout dev` could be miconstrued
as checking out the dev _directory_ rather than the branch...

https://github.com/flutter/flutter/issues/14865

https://github.com/flutter/flutter/issues/14893

* Try to make transitioning channels actually work.

* Update upgrade.dart

Generate the "version" file from git tags.
Remove the old VERSION file and mentions of versions in pubspec.yaml files.
Replace the old update_versions.dart script with a new roll_dev.dart script.
Update "flutter channel".
Update "flutter upgrade", including making it transition from alpha to dev.
Update "flutter --version" and "flutter doctor".

* 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

* convert flutter commands to use ToolExit for non-zero exit code
* add convenience method throwToolExit
* print ToolExit message iff not null

* refactor _run to runCmd

* replace requiresProjectRoot getter with call to commandValidator

* replace requiresDevice getter with call to findTargetDevice

* trace command requires a debug connection, not a device

* inline androidOnly getter

* rename command methods to verifyTheRunCmd and runCmd

* move common verification into BuildSubCommand

* rename deviceForCommand to device

* rename methods to verifyThenRunCommand and runCommand

* add flutter channel command
fixes https://github.com/flutter/flutter/issues/2513

* fix test

* address comments

* remove unused imports

* fix lints