Use pub_semver package in pub.

Use pub_semver package in pub.

R=nweiz@google.com

Committed: https://code.google.com/p/dart/source/detail?r=40849

[Bob Nystrom, 2014-09-25 22:30:40]

rnystrom@google.com changed reviewers:
+ nweiz@google.com

[Bob Nystrom, 2014-09-25 22:30:41]

The one missing piece is a separate patch to vendor in the package once it's up
on github. The only changes here are:

- Remove version and tests from pub.
- Change the import of version.dart to pull from the package.
- Lowercase "p" in "antiprioritize".

[nweiz, 2014-09-25 23:02:23]

lgtm, although obviously don't commit this until the GitHub package exists and
is vendored in.

[Bob Nystrom, 2014-10-01 17:57:30]

Committed patchset #1 (id:1) manually as 40849 (presubmit successful).