Fix suggested SDK constraints for the ^ operator.

This gets rid of the suggested prerelease suffix for SDK constraints
for pubspecs using the ^ operator, since 1.8.0 itself is out now and
probably no one is using a prerelease version of it.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//937503002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@43823 260f80e4-7a28-3924-810f-c04153c831b5