[JavaScript] Division operator should not be allowed after arrow functions

[Thom1729]

From #1600.

Simple example:

x=>{}
/a/g;

The second line will be interpreted as a division expression, but it should actually be a regexp literal. The reason for this is that arrow function literals are in a weird place in the grammar — in AssignmentExpression, “below” the comma operator but “above” most other binary operators.

The existing behavior is correct for the two following examples:

x=>x
/a/g;

(x=>{})
/a/g;

This bug is obscure and unlikely to come up in practice, and several other parsers get it wrong too. And I suspect that fixing it might add more complexity than it's worth. But it is a bug, so here's a bug report.

[Thom1729]

And as soon I write this up, I find that I wrote it up two and a half years ago in #2144. Closing as duplicate.