Load parallel transformers in the same isolate.

Transformers that are used in the package that defines them will be loaded in
the same isolate only if they're in the same phase. Otherwise, all transformers
within a given package will be loaded in the same isolate.

A future CL will take more advantage of this by more aggressively determining
when transformers can be safely loaded in parallel.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//323263002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@37296 260f80e4-7a28-3924-810f-c04153c831b5