Support ES6 MethodDefinition computed keys

Support ES6 MethodDefinition with computed keys.
Test cases are added.

https://code.google.com/p/esprima/issues/detail?id=563
I've updated #281. #281 introduces StringMap to clean up these code. It depends on this #279 and #280 PR.
In #281, I've created the `validateDuplicateProp`, it cleans up these duplicate code. PTAL :)