You take a bus to the Magic Kingdom, then take the monorail to the Polynesian. You can also take the express monorail to the TTC and walk to the Poly from there, if the express is running that early.
Bus service begins around 6:30 (i.e. the buses start rolling out of the depot around that time), and probably the first bus will hit All Stars at around 6:45. The bus service is not as regular before park opening, but there will probably be buses every 30-40 minutes until about an hour before opening, at which point they'll start coming more often. The bus ride will be 20 minutes or so to the MK, plus possible stops at other All-Stars hotels. Then you should allow 20 minutes to get to the Poly on the monorail, including waiting time.
Working that backwards, you need to be waiting for a monorail by 8:20, which means you need to be arriving at the MK bus stop by 8:10, which means you need to be on a bus by 7:40, which means you should be at the bus stop by 7-ish, since buses might be as far apart as 40 minutes that early in the morning.
Alternatively, you could just get a cab at about 8:15. It'll be about $20-25 to the Poly.