assert.rejects can error if a fat-arrow function is used

[smcclure15]

Tell us about your runtime:

• QUnit version: 2.16.0
• Which environment are you using? (e.g., browser, Node): Node
• How are you running QUnit? (e.g., QUnit CLI, Grunt, Karma, manually in browser): CLI

When the reject "reason" is a string, both function handlers and "fat-arrow" functions work well (both of these pass):

QUnit.test('String reason, function handler', assert => {
    let p = Promise.reject('reason');
    assert.rejects(p, function(err) {
        return err.toString().includes('reason');
    });
});
QUnit.test('String reason, arrow handler', assert => {
    let p = Promise.reject('reason');
    assert.rejects(p, err => {
        return err.toString().includes('reason');
    });
});

TAP version 13
ok 1 String reason, function handler
ok 2 String reason, arrow handler
1..2
# pass 2
# skip 0
# todo 0
# fail 0

However, when the "reason" is an Error object, the function handlers work well, but the fat-arrow's do not:

QUnit.test('Error reason, function handler', assert => {
    let p = Promise.reject(new Error('reason'));
    assert.rejects(p, function(err) {
        return err.toString().includes('reason');
    });
});
QUnit.test('Error reason, arrow handler, sync', assert => {
    let p = Promise.reject(new Error('reason'));
    assert.rejects(p, err => {
        return err.toString().includes('reason');
    });
});
QUnit.test('Error reason, arrow handler, async', async assert => {
    let p = Promise.reject(new Error('reason'));
    await assert.rejects(p, err => {
        return err.toString().includes('reason');
    });
});

If I run that as written, I see

TAP version 13
ok 1 Error reason, function handler
Error: Process exited before tests finished running
Last test to run (Error reason, arrow handler, sync) has an async hold. Ensure all assert.async() callbacks are invoked and Promises resolve. You should also set a standard timeout via QUnit.config.testTimeout.

So it barfed on the second test. If I skip the second one, I get:

TAP version 13
ok 1 Error reason, function handler
ok 2 # SKIP Error reason, arrow handler, sync
not ok 3 Error reason, arrow handler, async
  ---
  message: "Promise rejected during \"Error reason, arrow handler, async\": Function has non-object prototype 'undefined' in instanceof check"
  severity: failed
  actual  : null
  expected: undefined
  stack: |
    TypeError: Function has non-object prototype 'undefined' in instanceof check
        at Function.[Symbol.hasInstance] (<anonymous>)
  ...
1..3
# pass 1
# skip 1
# todo 0
# fail 1

I'm not sure if that sync/async behavior is related, or is a separate issue.

The error message is reminiscent of #1389, and likely a similar fix should resolve it.