Well, all the easy ones, anyway.

For some reason `// ignore:` isn't working for me so I've disabled
lints that need that. Also disabled those that require a ton of work
(which I'm doing, but not in this PR, to keep it reviewable).

This adds:
- avoid_init_to_null
- library_names
- package_api_docs
- package_names
- package_prefixed_library_names
- prefer_is_not_empty
- sort_constructors_first
- sort_unnamed_constructors_first
- unnecessary_getters_setters

This patch switches us from using our previous hacking approach to integrating
with package:test to using the new PlatformPlugin interface.

rename to printTrace(), printStatus(), and printError()

Fixes #311

...by requiring a new version of analyzer.

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

When the engine dies unexpectedly during test execution, we have to
terminate any tests running in that engine. Previously, they would just
hang. For some reason that I was never able to satisfactorily explain,
the WebSocket doesn't die in a way I can detect in this case. So
instead, we hand in a future that we only complete when we detect the
server subprocess ends.

The previous attempt to port the 'test' framework to the new framework
wasn't super-successful. This does a better job, hopefully.

This command uses package:test to run Dart tests with sky_shell.  For this to
work, we need https://github.com/dart-lang/test/tree/hacky-loader-hook to land.
We're also not smart enough to find sky_shell ourselves yet. Instead, we take
the path as input using an environment variable. Eventually, we'll be able to
get the sky_shell executable from package:sky_engine, but we don't yet ship
that executable.