Have ControlFlowAnalyzer record fields that are written.  Previously,
it only recorded fields that were read.  This allows fixing a bug
in CodeSplitter where a field could end up being written before the
field itself was loaded.

Also, this adds a test case for runAsync.

Review by: kprobst



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@4358 8db76d5a-ed1c-0410-87a9-c151d255dfc7