* 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
* convert pubGet to throw ToolExit on non-zero exit code
* convert commandValidator to throw ToolExit for non-zero exit code
* convert flutter commands to throw ToolExit for non-zero exit code
* use convenience method throwToolExit
* only show "if this problem persists" for unusual exceptions
* 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
This change adds a top-level getBuildDirectory func and funcs for
android, aot, asset, ios build products.
Developers may now add a "build-dir" mapping to their
~/.flutter_settings (JSON format) config file. Output directory is
relative to the main flutter application directory.
This change also changes the default build directory for iOS builds to a
subdirectory of the configured build directory, 'build/ios' by default.
Host tools can be found in the artifact cache directory for the host platform.
If a developer wants to use a local engine build instead, then provide an
--engine-build flag that selects the specific engine build variant.
