Patch for interim state of incremental compile.
If a user manually switches GwtAstBuilder on/off, cached units may
not have the AST partial data.

Review at http://gwt-code-reviews.appspot.com/1464804


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10388 8db76d5a-ed1c-0410-87a9-c151d255dfc7