SpoutDev/Spoutcraft — comment on commit 0808e8aa7d7d4fb620fc402f9bdcdab80dcc0fde
File: src/minecraft/org/spoutcraft/client/gui/singleplayer/GuiCreateWorld.java line 245
By zml2008:

Note the since 1.6 part. iirc Spoutcraft is supposed to be 1.5-compatible.