CompilationState now keeps a Collection of units rather than a Set; this allows us to simply reuse the map's values().

This replaces code that used to construct a new HashSet each time new units were generated, which was super-linear.  It turns out none of the callers rely on the the return type being a Set.

Review by: jat (desk)

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@5346 8db76d5a-ed1c-0410-87a9-c151d255dfc7