Fix arrow function parameter strictness

Per ES6 Draft 21, section 14.2:

  ArrowFormalParameters:
    ( StrictFormalParameters )

Per 14.1.1:

  StrictFormalParameters : FormalParameters
    It is a Syntax Error if BoundNames of FormalParameters contains any duplicate elements.

  FormalParameters : FormalParameterList
    It is a Syntax Error if IsSimpleParameterList of FormalParameterList is false and BoundNames of FormalParameterList contains any duplicate elements.
    It is a Syntax Error if IsSimpleParameterList of FormalParameterList is false and BoundNames of FormalParameterList contains either ′′eval′′ or ′′arguments′′.

https://code.google.com/p/esprima/issues/detail?id=472