Adds a --private-key option to the build command, which specifies an ECDSA
private key. When this is provided along with a manifest, the manifest is
prepended to the .flx package and signed with the private key. The manifest
also includes a SHA-256 hash of the zipped content portion of the .flx package.

This is used by the Flutter updater package, to verify that updates are
from the right publisher.

Fixes https://github.com/flutter/engine/issues/1613

The `run_mojo` command doesn't integrate with `FlutterCommand` and doesn't
understand how to download its toolchain components ahead of time. Eventually
we should teach `run_mojo` how to integrate with the `Toolchain` class, but
until then, we can fix the regression by eagerly setting
`ArtifactStore.packageRoot` again.

Fixes https://github.com/domokit/mojo/issues/475

We were checking for a pubspec.yaml too early.

Now we use a debuggable APK and chmod the installation directory to be world
readable/writable.

Fixes https://github.com/flutter/engine/issues/126

Instead, just print the usage.

A common use case for members of the Flutter team is to have a dependency
override for the flutter package that points back into the engine src tree.
We can use that override to automatically detect the engine src path, which
makes the command line shorter.

Instead of calling through `pub` to invoke build, this patch refactors the
build command so that it can be called directly.

Previous we dumped a stack trace. Now we produce a more sensible error.

This patch adds a couple print statements to explain why the first run of
`flutter start` takes a while. (We need to download the APK and install it on
the device.)

This patch improves the error handling for several arbitrary issues I
encountered while experimenting with the tool this evening.

This patch makes `flutter start` work without a clone of the engine git
repository. Making this work pulled a relatively large refactor of how the
commands interact with application packages and devices. Now commands that want
to interact with application packages or devices inherit from a common base
class that holds stores of those objects as members.

In production, the commands download and connect to devices based on the build
configuration stored on the FlutterCommandRunner. In testing, these fields are
used to mock out the real application package and devices.

Also fixes some minor bugs with iOS and Android interactions.

The --install command isn't needed anymore. We automatically detect when
we need to install the APK.

Expose the main entry point for the tools via the library lets us run the tools
from the Flutter package, which simplifies the setup for end developers because
they don't need to declare a dependency on sky_tools directly.

Move Command subclasses into commands directory

Changing Desc to match Github Desc.

I'm trying to get a feel for the code by writing some simple cleanup patches.

Get iOS devices working on Mac if you specify sky-src-path.

Fix Flutter project template

Add implementation of start and stop commands for iOS.

Don’t give a scary error about not being able to run on Android if there’s no Android device connected.