Fix debounceEvent

[f111fei]

debounceEvent is abnormal sometimes.

Steps to Reproduce:

Here is my test case:

test('Debounce Event', function () {
    let doc = new Samples.Document3();

    let onDocDidChange = debounceEvent(doc.onDidChange, (prev: string[], cur) => {
        if (!prev) {
            prev = [cur];
        } else if (prev.indexOf(cur) < 0) {
            prev.push(cur);
        }
        return prev;
    }, 10);

    onDocDidChange(keys => {
        assert.ok(keys, 'was not expecting keys.');
        if (keys.length === 3) {
            doc.setText('4');
            assert.deepEqual(keys, ['1', '2', '3']);
        } else {
            assert.deepEqual(keys, ['4']);
        }
    });

    doc.setText('1');
    doc.setText('2');
    doc.setText('3');
}); 

1. Insert test code to event.test.ts.
2. run scripts\test.

Here is error:
was not expecting keys.

Reason:
Some code is changing output during emitter.fire

Solution:
Reset output before emitter.fire

[jrieken]

Some code is changing output during emitter.fire

not sure why you mean by that? Do you have two listeners and one modifies the event object?

[jrieken]

Oh, I see - you trigger event while processing them... Can you pls add you test to this PR

[f111fei]

@jrieken it's here bec3066

[jrieken]

since this test is async (setTimeout) you must add the done parameter and call it. Alternatively make this test return a promise.

[f111fei]

@jrieken it's ok? a7f47fa

[jrieken]

yeah - looks good. waiting for the builds to finish

[jrieken]

@f111fei - sorry we had some hiccups today. Can you rebase with master and try again?

[f111fei]

@jrieken I do as you say but I'm not sure if I'm right.

[jrieken]

LGTM - thanks for the contribution