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: https://dart.googlecode.com/svn/branches/bleeding_edge@45513 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/solver/backtracking_solver.dart

@@ -395,6 +395,7 @@ class BacktrackingSolver {
       // Don't get stuck in cycles.
       if (visited.contains(package)) return;
       visited.add(package);
+      dependers[package].forEach(walk);
     }
 
     walk(dependency);