Support new optional parameter syntax (issue 4289)

Support new optional parameter syntax (issue 4289)
Committed: https://code.google.com/p/dart/source/detail?r=10832

[scheglov, 2012-08-16 16:30:58]

LGTM

Does it work when all - formal, optional and named parameters declared?

[Brian Wilkerson, 2012-08-16 16:38:14]

> Does it work when all - formal, optional and named parameters declared?

It isn't possible to have both optional and named parameters in the same
parameter list.

formalParameterList:     
      '(' ')' 
    | '(' normalFormalParameters ( ‘,’ optionalFormalParameters)? ')'
    | '(' optionalFormalParameters ')'
    ;

optionalFormalParameters:
     optionalPositionalFormalParameters |
     namedFormalParameters
    ;