code: public List<JCCompilationUnit> enterTreesIfNeeded(List<JCCompilationUnit> roots) {
         if (shouldStop(CompileState.ATTR))
             return List.nil();
          return enterTrees(roots);
      }
docs: Enter the symbols found in a list of parse trees if the compilation
  is expected to proceed beyond anno processing into attr.
  As a side-effect, this puts elements on the "todo" list.
  Also stores a list of all top level classes in rootClasses.