Ah, I didn't mean to get rid of the mention of undefined in the message -- sorry! What do you think of:

```
(this.constructor.displayName || 'ReactCompositeComponent') +
'.shouldComponentUpdate(): Returned undefined instead of a ' +
'boolean value. Make sure to return true or false.'
```

(And FB style guide says single quotes.)

Otherwise I think this looks great now. Do you mind squashing your commits into one (using `git rebase -i`)? You'll need to force-push to your branch but GitHub will update the PR automatically. After that I'll let @zpao double-check and merge.