ES6 MethodDefinition computed keys

issue: https://code.google.com/p/esprima/issues/detail?id=563

MethodDefinitions can also have computed names, via the following productions:

```
MethodDefinition : PropertyName ( StrictFormalParameters ) { FunctionBody }
MethodDefinition : get PropertyName ( ) { FunctionBody }
MethodDefinition : set PropertyName ( PropertySetParameterList ) { FunctionBody } 
...
PropertyName : ComputedPropertyName 
```

Given the nature of computed property names (evaluated at run-time) they also cannot participate in the duplicate member name checks.