We use a number of non-public APIs in the test package, which makes our
dependency quite fragile. This patch pins a specific, known-good version. We
should update to the lastest version in a follow-up patch.

Using local paths ensures that each package sees each other package at HEAD.

...by requiring a new version of analyzer.

Also, when the subprocess is terminated unexpectedly, report the actual
problem in more detail.

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

remove an unused import

review comments

rename st --> stack

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.

remove a runInShell arg on windows when launching pub

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.

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.