Run pub tests against older versions of barback.

If we're going to claim to support older versions of barback
indefinitely, we need to test that support. This will become
increasingly important as the glue code for more recent versions
becomes more and more distinct from that for older versions.

This does slow down the time it takes to run all the pub tests by
about 20%. It may be worth considering only running a subset of
barback tests against all versions when running locally.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//291843011

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@36707 260f80e4-7a28-3924-810f-c04153c831b5