Remove copy of unittest code

[kevmoo]

Waiting on a clean model from the core lib. At the moment, one needs to hard-wire paths to the local install of the SDK, which is just asking for trouble.

CC @prujohn

[munificent]

Before long you will be able to reference packages in the SDK from pub. You can actually do that now, but there's no clean way to tell pub where your SDK install is so it's a bit, uh, limited in functionality. Once we have pub reading an environment variable for it, you'll be able to add:

dependencies:
  unittest:
    sdk: unittest

to your pubspec and then just:

$ pub install

in order to use the one you have locally installed. I added this bug to track this so you can know when it's supported.

[kevmoo]

Done!

b6c1e35