Big refactoring of test code and api-checker to remove dependencies on CompilationUnit.

This is in preparation for more refactoring, which will make CompilationUnit stateless outputs rather than stateful inputs/outputs.

Review by: bobv, amitmanjhi, rjrjr

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@6783 8db76d5a-ed1c-0410-87a9-c151d255dfc7