Ability to run tests that require '--pub-serve'

[greglittlefield-wf]

When testing code that is modified by Pub transformers, pub run test needs to be run with the --pub-serve option. See: https://github.com/dart-lang/test#testing-with-barback.

Right now, there's not a way to pass in that flag.

---

What would be really awesome is if --pub-serve support was be integrated, and dart_dev also spun up a Pub server automatically before running the tests.

The test package has a utility it uses in its tests that could probably be leveraged as a starting point.

That's a little more involved, though...

[greglittlefield-wf]

Closing this; fixed in #92.