Electronic flasher with low load detection
In the old days of thermal flashers used in automotive applications, the faster flashing when the load was reduced (due to a burned out bulb) was a by-product of the way they worked (less current meant that the bimetal temperature overshot less, so they flashed faster). This is considered a useful feature now, but it's not "natural". 

One method would be to use a microcontroller and measure the current (very crudely is good enough). Something like a 6-pin or 8-pin PIC would do the job. Timing accuracy will be more than good enough using the internal clock. 

Here is an example of a commercial flasher that uses a current measurement shunt. You can see the DIP chip hiding underneath the relay. The current measurement is by measuring voltage across the red-circled "M"-shaped stamped piece of metal (which is a very low value resistor). The author of this web page modified the flasher so it would not interpret the lower current of an LED retrofit bulb as an incandescent bulb that had failed. 



Edit: 
If you want to use a PRC ASIC, you should do a proper market search, but here is an example one: 



You should use about 110K for R1 to get the desired 85cpm flash frequency. The threshold for current detection is typical 51mV, so the voltage drop is quite low.