Add a Pub source that checks packages out from Git.

Add a Pub source that checks packages out from Git.

This was submitted as r7623, but was then reverted due to test failures. The new
version doesn't depend on Git's output.

Committed: https://code.google.com/p/dart/source/detail?r=7629

[nweiz, 2012-05-14 20:38:26]

The only change from last time is the RegExp for validating Pub's output for the
Git tests in pub_test.dart.

[Bob Nystrom, 2012-05-14 21:32:22]

lgtm