The dragon egg does not spawn if there already is an end gateway.

Steps to reproduce

1. Create a new world
2. Go to the End
3. Place an end gateway (for example using the command /setblock ~ ~100 ~ end_gateway to make sure that the dragon does not escape through it)
4. Leave the world (or the server) and open it again (or rejoin)
5. Kill the ender dragon. Note that no egg is spawned.

The egg will spawn if you skipped step 3 or step 4.

Original report

If you are trying to Spawn a Dragon for the first time and you have an End Gateway around for some reason already, it will spawn a Dragon which does not behave like the first one. I think changing the detection for endPortals to something like this is going to be sufficient.

if (tileentity instanceof TileEntityEndPortal && !(tileentity instanceof TileEntityEndGateway)) {...}

instead of

if (tileentity instanceof TileEntityEndPortal) {...}