Add listen command and basic test, and don’t do unnecessary repeated work when listening or poking the android server.

Add optional prefix to runCommandAndStreamOutput for logs commands.

Also add unchecked runSync wrapper and use it in places where command failures don’t matter.

Add stop command and supporting Android support.

Fix sky_tools build command to look for package-root in global args

Also refactors the command line assembling to make a bit more sense.

Fixes #53

This makes the 'package-root' option universal for sky_tools and configures the
ArtifactStore with it statically at startup. The actual sky_engine revision
is computed on demand.

run_mojo: Add --mojo-release and --mojo-debug flags.

The flags control which build of mojo to run (Debug or Release).
Default is Release.

Make cached artifact location mirror local builds more closely

Refactor all the commands to be Commands from the Args package.  Also use CommandRunner for the top-level command.

Refactor all the commands to be Commands from the Args package.  Also use CommandRunner for the top-level command.

sky_viewer.mojo no longer needs a window manager embedding as of
sky_engine 0.0.27, so this updates the sky_tools run_mojo command so it works
again. This really should be expressed in the pubspec dependencies to avoid
broken combinations but that needs to happen on the sky side.

Forward additional arguments to runner in run_mojo command

fix a path issue when creating new flutter apps

This adds the following commands to sky_tools:
  sky_tools cache clear: Nukes all local artifacts in the cache
  sky_tools cache populate: Populates the cache with all known artifacts

This is useful both to fix busted caches and to make sure that the cache is
fully populated so that subsequent operations can proceed without needing
network access.

This teaches commands that need binary artifacts to explicitly instantiate an
instance of the ArtifactStore with the appropriate packageRoot string. The
ArtifactStore can then remember the package root and compute the engine
revision when created and remember those for subsequence calls.

Teach sky_tools mojo_run to run on Android (using mojo devtools)

This teaches sky_tools mojo_run --android to invoke mojo's devtool's mojo_run
script with the right flags for invoking sky_viewer on android. This tells the
devtools script to load sky_viewer.mojo from https://storage.googleapis.com/...
and to load app.flx (or whatever the developer specifies as --app) from the
filesystem using the devtools http server.

`dart bin/sky_tools.dart --debug --sky-src-path=/path/to/sky/src/ install` now works.