Switch to sets for constants in visitIdentifierNodes.js
Switch to sets for constants in visitIdentifierNodes.js

Node 4 doesn't support Array.includes, so we need a different mechanism
of finding strings in an array. Sets have the `has` function, and it
makes sense to use sets here.

Galooshi/import-js,trotzig/import-js,trotzig/import-js,trotzig/import-js