I'm surprised we haven't seen more of this; I suspect it's because the
mixed `module.exports=` + `export.foo=` pattern isn't that common.
However, it'll happen any time that the exported symbol is unknown;
getCommonJsExportEquals blithely clones unknownSymbol and proceeds to
stick the `exports.foo=` properties onto it.
This causes problems later, because the compiler checks for
unknownSymbol with `===`. The fix is to not stick properties onto a
clone of unknownSymbol. This makes the correct errors appear and removes
the crash.
