Fix suggested SDK constraints for the ^ operator.

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

Committed: https://code.google.com/p/dart/source/detail?r=43823

[Bob Nystrom, 2015-02-17 21:31:41]

LGTM.

Is there a test that includes this verbiage that also needs to be updated?

[nweiz, 2015-02-17 21:51:55]

On 2015/02/17 21:31:41, Bob Nystrom wrote:
> LGTM.
> 
> Is there a test that includes this verbiage that also needs to be updated?

No, I knew this change was coming when I wrote the tests so they're resilient to
it.

[nweiz, 2015-02-17 21:52:28]

Committed patchset #1 (id:1) manually as 43823 (presubmit successful).