When tightening method calls, use a more reliable technique to
determine what methods a given method overrides.  Use the
override chains determined when JTypeOracle is first built,
rather than trying to compare type signatures of methods.

Review by: scottb


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@3534 8db76d5a-ed1c-0410-87a9-c151d255dfc7