increase test timeout

Stop requiring customers to use a dev SDK

Modify run_mojo command to take path to devtools.

Generalize path handling in flutter init

Made changes as discussed.
R=@jamesr

In case we run into trouble, this at least gives us an escape hatch.

This will make "-o ." work as expected to initialize a project in the
current directory

Wait for sky server to start before starting device

Instead of just waiting for the sky server process to start
before we start the activity on the device, this causes us to
wait for the sky server to actually start listening on its port

Fixes #141

Depend on exact version of flx package so I can make breaking changes.

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.