* 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
* 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 google analytics
* send in the run target type
* track device type targets
* use the real GA code
* review comments
* rev to usage 2.0
* rev to 2.2.0 of usage; add tests
* review comments
This command explicitly populates the flutter tool's cache of binary artifacts.
Also, teach `flutter create` to update the cache in case its the first command
that a user runs.
