Stop building (unused) unlinked summaries for packages.
Improves update speed considerably (for `n` packages it saves us `n` needless calls to `pub get`).
* 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
* 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
1) Moved basic utility code into base/ directory to make it clear which code
doesn't depend on Flutter-specific knowldge.
2) Move the CommandRunner subclasses into a runner/ directory because these
aren't commands themselves.
