Named exports can have optional module specifier.

This is basically implementing

  ExportSpecifierSet ::=
    "{" (ExportSpecifier ("," ExportSpecifier)* ","?)? "}"
    ("from" ModuleSpecifier)?

https://code.google.com/p/esprima/issues/detail?id=410
Distinguish between default vs named import specifiers.

https://code.google.com/p/esprima/issues/detail?id=410
Allow ReservedWord to be used as export and import names.

https://code.google.com/p/esprima/issues/detail?id=410