assert: support arrow functions in .throws()
x instanceof f where f is an arrow function throws a (spec-conforming)
"Function has non-object prototype 'undefined' in instanceof check"
exception.
Add a workaround so that it's possible to pass arrow functions as the second argument to assert.throws(). The try/catch block is a little jarring but swapping around the clauses in the if statements changes the semantics too much.