Fix version solving in pub.
Fix version solving in pub.

I deleted a line in r45508 because it had an unused variable, but it
turned out that it also had relevant side-effects.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1123473005

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@45513 260f80e4-7a28-3924-810f-c04153c831b5

computmaxer/pub,dart-lang/pub,cjkao/dart-pub,maxwellpeterson-wf/pub,dart-lang/pub