Could the dinosaurs have seen the asteroid that killed them? Wikipedia says the Chicxulub impactor is thought to have been a 10-15 km diameter object. Would it have been visible to a (human*) naked eye before impact? And if so, would it have appeared like a star that grew brighter and brighter each night?

* I know, there were no humans at the time.
The answer is yes; for a few nights prior to the impact (assuming they had eyes with a similar sensitivity to our own and could look up!). It could also be much longer than this if the object was a cometary body.
Details:
Impacting solar system objects would have relative closing speeds from around 11 to 72 km/s.
We could take the optimal case that the asteroid approaches whilst fully lit by the Sun (which probably precludes the minimum and maximum speed in the range quoted above) and then scale from another similar body - say the asteroid Vesta. This has a diameter of around $a=520$ km, gets as close as $d=1.14$ au from the Earth and has a maximum brightness of about $m=5.2$ apparent magnitude (and is hence just visible to the naked eye) and an observed flux $f = f_0 10^{-0.4m}$, where $f_0$ is a zeropoint for the magnitude scale.
Thus the flux $f_n$ received by a near-Earth asteroid of diameter $a_n$, at a distance $d_n$ from Earth (in au) and with the same reflectivity would be
$$ f_n = f\left(\frac{a_n}{a}\right)^2 \left(\frac{1+d}{1+d_n}\right)^2
\left(\frac{d}{d_n}\right)^2$$
The magnitude of the dinosaur killer would then be
$$m_n = m -2.5\log (f/f_n)$$
To be an at all conspicuous naked eye object, $f_n \geq f$. If we assume the dinosaur-killer had $a_n=10$ km, then
$$ d_n^2(1+d_n)^2 \leq 0.0022$$
An approximate solution is obtained by assuming $d_n \ll 1$ and thus we find
$ d_n \leq 0.047$ au or 7 million km.
Moving at say 30 km/s, then it gets closer by 2.6 million km per day, thus hitting the Earth about 3 days after becoming a naked eye object. Obviously this would be longer for a slower approach speed or for a larger or more reflective asteroid. But shorter for a smaller, faster asteroid or if the asteroid approached from a direction not fully illuminated by the Sun or had a smaller albedo than Vesta.
Another possibility is that the object is of a cometary origin with an icy composition. If that were so then it could be much brighter as a result of sublimation, outgassing and having a bright cometary nucleus and tail. he answer would still be yes, but the visibility period could be weeks (comets are rather unpredictably bright).
It thus seems to me that there is a plausible range of parameters and trajectories where a dinosaur-killing asteroid could be observed and then observed to grow brighter over a few nights, but probably not much longer than that unless it was a comet.