Map#get needs a default value param?

[Brendan Eich]

Tab Atkins Jr. wrote:
> This isn't too bad, and we can always wait for the language to expand
> with subtypes of Map that handle this behavior better (or use a
> library), like the aforementioned Counter and DefaultDict of Python.

Previously: 
https://mail.mozilla.org/pipermail/es-discuss/2012-January/019723.html 
et seq.

/be