Made changes as discussed.
R=@jamesr

remove an unused import

review comments

rename st --> stack

Add a badge for AppVeyor.

(TBR)

We also have to require a newer test package, since the old one depends
on a version of the analyzer that uses dart:profiler, which is gone and
replaced by dart:developer.

Increase the test timeout for the `init` test to 2 minutes (test was timing out downloading assets on appveyor).

remove a runInShell arg on windows when launching pub

Don't rely on external shasum program to calculate sum of the APK.

Windows has no direct way to kill a process based on port. Uses netstats and loops through the results to find the correct process to kill.

Also modify Process.run for the server to runInShell if on Windows.

Style nits.

Avoid script snapshot creation if the --precompiled flag is set during builds

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.)