Plastic fitting cracked in kitchen faucet supply line
I tried heat-shrink tubing: no way, not enough of a seal for the pressure.  I tried electrical tape: much better, but could not withstand the 30 psi max pressure of a supply tube.

The tube itself is, unfortunately, part of the faucet assembly, so I was forced to replace the entire faucet.  I notice that Pfister now uses a different fitting for the quick connect joint, no doubt because the one depicted above was breaking in the field.

So the best answer seems to be, sadly, to replace the entire faucet assembly.