RFR: JDK-8231311: javac throws NPE in TransTypes.retype

[Vicente Romero]

Please review fix for [1] at [2]. The fix is ending the compilation at 
TransTypes if after erasing the type of a method symbol, its type is not 
a method type and thus there is no way for the compilation to continue.

Thanks,
Vicente

[1] https://bugs.openjdk.java.net/browse/JDK-8231311
[2] http://cr.openjdk.java.net/~vromero/8231311/webrev.00/