Removes CompilationUnit.isCompiled().

- CompilationState retains a full map of all units, including error units.
- Only transitively valid units are passed into TypeOracle.

http://gwt-code-reviews.appspot.com/759801/show
Review by: kplatfoot


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8525 8db76d5a-ed1c-0410-87a9-c151d255dfc7