Introduces a new Dart analysis wrapper that works directly with the analyzer API (in favor of shelling out to a separate process).

Some consequences:

  * we no longer need to fear parts (simplifying our dart file gathering)
  * we can filter by error code (when needed), rather than by error strings
  * no more IO scraping
  * no need to generate `main()` or to run with `--package-warnings`
  * we now specify an analyzer (and linter) version in the pubspec (we’ll want to make sure this doesn’t diverge too far from the analyzer shipped with the SDK but it does give us some room to play with experimental builds)
  * no more (re)scanning of error source files (and so no more source cache)
  * should generally be a bit simpler and easier to maintain
  * runs a bit faster :)

We need to pin the version of package:analyzer we use to avoid version skew
within our project.

The mojo_sdk has cleaned up its dependencies, so we no longer need to depend on
package:test from package:flutter.

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

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.

This package contains WidgetTester, which is very useful when writing tests for
widgets.