SpiderMonkey complains about functions not always returning a value. In
most cases that is a conscious code style choice, so it is not fixed
here.
In one case (MockDate) the interpreter thought you could have fallen off
the end of a "switch" statement, although the number of arguments
prevented that. This was fixed by changing the last case to "default".
In another case (QueueRunner) the function really did return a value
sometimes and nothing other times, although as far as I could see, it
could only ever return "undefined". The function now explicitly only
returns no value.
See #751
