FL5150 (LED dimmer) circuit explanation
When the current passes through the MOSFET, the mid-point of both the
  MOSFETS is in direct line of AC supply.


No it isn't. It is a node with a GND symbol on it and this doesn't automatically mean that it connects to true ground of the incoming AC supply. If this happened, the MOSFETs would become ashes in milliseconds.

The ground symbol is just a way of hinting that the GND pin on the chip is connected to the centre-point of the MOSFETs. It connects nowhere else and it certainly should not be assumed that it connects to earth.


  How is this thing working? Is the voltage on the mid-point of both the
  MOSFETs at a constant voltage?


Yes it is relative to the VS pin on the chip. Regard the two MOSFETs' internal diodes as being the lower half of a bridge rectifier. D1 and D2 form the upper half. R1 and C2 smooth the voltage for the chip.