Use custom block populators with default chunk generator

// this is a clone of CraftServer.createWorld, amended to accept extra block populators
public World createWorldWithPopulators(WorldCreator creator, Collection<BlockPopulator> extraPopulators)
{
    if (creator == null) {
        throw new IllegalArgumentException("Creator may not be null");
    }

    CraftServer craftServer = (CraftServer)plugin.getServer();
    MinecraftServer console = craftServer.getServer();

    String name = creator.name();
    ChunkGenerator generator = creator.generator();
    File folder = new File(craftServer.getWorldContainer(), name);
    World world = craftServer.getWorld(name);
    WorldType type = WorldType.getType(creator.type().getName());
    boolean generateStructures = creator.generateStructures();

    if (world != null) {
        return world;
    }

    if ((folder.exists()) && (!folder.isDirectory())) {
        throw new IllegalArgumentException("File exists with the name '" + name + "' and isn't a folder");
    }

    if (generator == null) {
        generator = craftServer.getGenerator(name);
    }

    Convertable converter = new WorldLoaderServer(craftServer.getWorldContainer());
    if (converter.isConvertable(name)) {
        craftServer.getLogger().info("Converting world '" + name + "'");
        converter.convert(name, new ConvertProgressUpdater(console));
    }

    int dimension = 10 + console.worlds.size();
    boolean used = false;
    do {
        for (WorldServer server : console.worlds) {
            used = server.dimension == dimension;
            if (used) {
                dimension++;
                break;
            }
        }
    } while(used);
    boolean hardcore = false;

    WorldServer internal = new WorldServer(console, new ServerNBTManager(craftServer.getWorldContainer(), name, true), name, dimension, new WorldSettings(creator.seed(), EnumGamemode.a(craftServer.getDefaultGameMode().getValue()), generateStructures, hardcore, type), console.methodProfiler, creator.environment(), generator);

    if (craftServer.getWorld(name) == null) {
        return null;
    }

    internal.worldMaps = console.worlds.get(0).worldMaps;

    internal.tracker = new EntityTracker(internal); // CraftBukkit
    internal.addIWorldAccess((IWorldAccess) new net.minecraft.server.WorldManager(console, internal));
    internal.difficulty = 1;
    internal.setSpawnFlags(true, true);
    console.worlds.add(internal);

    if (generator != null) {
        internal.getWorld().getPopulators().addAll(generator.getDefaultPopulators(internal.getWorld()));
    }
    internal.getWorld().getPopulators().addAll(extraPopulators); // use the extra block populators!

    craftServer.getPluginManager().callEvent(new WorldInitEvent(internal.getWorld()));
    System.out.print("Preparing start region for level " + (console.worlds.size() - 1) + " (Seed: " + internal.getSeed() + ")");

    if (internal.getWorld().getKeepSpawnInMemory()) {
        short short1 = 196;
        long i = System.currentTimeMillis();
        for (int j = -short1; j <= short1; j += 16) {
            for (int k = -short1; k <= short1; k += 16) {
                long l = System.currentTimeMillis();

                if (l < i) {
                    i = l;
                }

                if (l > i + 1000L) {
                    int i1 = (short1 * 2 + 1) * (short1 * 2 + 1);
                    int j1 = (j + short1) * (short1 * 2 + 1) + k + 1;

                    System.out.println("Preparing spawn area for " + name + ", " + (j1 * 100 / i1) + "%");
                    i = l;
                }

                ChunkCoordinates chunkcoordinates = internal.getSpawn();
                internal.chunkProviderServer.getChunkAt(chunkcoordinates.x + j >> 4, chunkcoordinates.z + k >> 4);

                while (internal.updateLights()) {
                    ;
                }
            }
        }
    }
    craftServer.getPluginManager().callEvent(new WorldLoadEvent(internal.getWorld()));
    return internal.getWorld();
}

public World createWorldWithPopulator(WorldCreator creator, BlockPopulator extraPopulator)
{
    ArrayList<BlockPopulator> extraPopulators = new ArrayList<BlockPopulator>();
    extraPopulators.add(extraPopulator);
    return createWorldWithPopulators(creator, extraPopulators);
}