ExportedNames incomplete for NameSpaceExport production

[spectranaut]

Hi @jdalton, I'm pretty sure this is a necessary addition to the ExportedNames algorithm, can you review?

Presently, the ExportedNames has a rule for the production NameSpaceExport here:

ExportFromClause : NameSpaceExport
    Return the ExportedNames of NameSpaceExport. 

But no way to further evaluate the ExportedNames of NameSpaceExport.

I believe, because of the chain of production (and the line, Return the ExportedNames of ExportFromClause above), we can replace these lines with:

NameSpaceExport : * as IdentifierName
    Return a List containing the StringValue of IdentifierName.

[jdalton]

Hi @spectranaut!

You are totally more familiar with the formal spec text process than myself. I used to know the ins and outs back in the day, but now with so much meta/shorthand/composition spec syntax it's hard for me to follow. The PR looks good to me, but I'll defer to @bterlson for the real review.