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
Is there any way we can remove the duplication with lines 4495-4511? Maybe just call a function with `('get', 'set')` and here with `('set', 'get')`?