Functions may incorrectly return a value instead of undefined (callback mode only)

[bjouhier]

Following test fails in callback mode because f returns 1:

asyncTest("return undefined", 1, function(_) {
    function read(_) {
        return delay(_, 1); 
    }
    function f(_) {
        read(_);
    }
    strictEqual(f(_), undefined);
    start();
});

[bjouhier]

Was due to an overly aggressive optimization of the transformed code: fn(function() { cb(); }) was simplified to fn(cb) in cases where it shouldn't have been.