Out of memory crash: World generation exhausts Java heap space

When creating a world using a redstone mechanism, the game crashed after about 24 hours with an out of memory exception.

Crash report is attached.

To reproduce:
1. Create a new world in creative mode (I used seed 1, gamerules: doWeatherCycle=false, doDaylightCycle=false, commandBlockOutput=false).
2. Unzip the attach file "g18.zip" into the "generated" folder of the save file.
3. Use a structure block to load the redstone mechanism "g18" into the spawn chunks of the Overworld.
4. Start the mechanism and let it run for a while, 24 hours or more. Eventually the game will crash.