@isaacs really strange here. If I change `_needTickCallback()` to `_tickCallback()` all but one tests passes:
```
assert.js:102
  throw new assert.AssertionError({
        ^
AssertionError: 1 == 2
    at process.<anonymous> (/var/projects/node-trevnorris/test/simple/test-process-next-tick.js:43:10)
    at process.EventEmitter.emit (events.js:124:20)
```
Can you help explain why that's happening?