* Added '--check-for-remote-artifacts' option for Flutter Doctor.
This option takes a Flutter engine revision and issues HEAD requests to
determine whether or not artifacts for the provided engine revision are
available from cloud storage. This functionality is needed for the Dart
SDK autoroller to avoid creating a PR to roll the engine into the
framework while artifacts haven't finished building, which would cause
Cirrus tests to fail.
* Flatten change out from lots of merges and temporary hacks for #7224
* Expand fakes to cover cases where doctor passes
* -v != --verbose
* Cosmetic touchups
* delint
* Clean up summary line, add a switch for review comments
* Review comments
* nit
* review comment
* review comments
* 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
