Review request for JDK-8073707: const re-assignment should not reported as a early error

[Hannes Wallnoefer]

Please review JDK-8073707: const re-assignment should not reported as a 
early error:

http://cr.openjdk.java.net/~hannesw/8073707/

This is Sundar's patch from two days ago with added runtime checks in 
codegen. I also rewrote the test, taking advantage of the fact that 
const reassignment no longer causes a SyntaxError.

Thanks,
Hannes