Uh.. this already works.  It was fixed in bug 17508.

Here's an example.  Say you have:

  enum Foo { "foo", }

The tokenizer sees the '{', then expects an EnumValueList.  

  EnumValueList	→	string EnumValueListComma

The tokenizer sees the string "foo", then expects EnumValueListComma.

  EnumValueListComma	→ "," EnumValueListString
                          | ε

The tokenizer sees a ',', then expects EnumValueListString

  EnumValueListString	→ string EnumValueListComma
                          | ε

The tokenizer doesn't see a string, but EnumValueListString is allowed to be empty so we match all that, go back to our Enum production, see the expected '}', and all is good.

Hm, dunno how I missed those | ε bits. Nevermind!

*** Bug 27701 has been marked as a duplicate of this bug. ***