Assert: Allow fat arrow functions in throws expected handler

[smcclure15]

fixes #1389
added positive and negative unittests

[Krinkle]

It took me a while to figure out why this works. In particular, I was trying to find how absence of this condition leads to TypeError: 'prototype' property of x is not an object.

But I see now that the issue is coming from the native code behind the instanceof operator. I... wasn't expecting that!

arr = () => {};
function Foo() {}

x = new Foo();
x instanceof arr;
//> TypeError: 'prototype' property of arr is not an object

[Krinkle]

LGTM. I'll get review this once more tomorrow and then land it.

Could you add an inline comment about the instanceof caveat for future eyes?

[smcclure15]

Agreed, this also took me a bit.
Kudos to @ohze for supplying the solution in their issue.
This was indeed a "Good first issue" - glad to help!