Okay, back to arbitrary offsets!

src/main/java/net/minecraft/server/WorldMap.java

@@ -37,6 +37,7 @@ public class WorldMap extends WorldMapBase {
         // CraftBukkit start
         byte dimension = nbttagcompound.c("dimension");
 
+        if (dimension >= 10) {
             this.worldUID = nbttagcompound.getLong("WorldUID");
             CraftWorld world = (CraftWorld) server.getWorld(this.worldUID);
             // Check if the stored world details are correct.
@@ -47,6 +48,7 @@ public class WorldMap extends WorldMapBase {
             } else {
                 dimension = (byte) world.getHandle().dimension;
             }
+        }
 
         this.map = dimension;
         // CraftBukkit end

src/main/java/org/bukkit/craftbukkit/CraftServer.java

@@ -393,7 +393,7 @@ public final class CraftServer implements Server {
             converter.convert(name, new ConvertProgressUpdater(console));
         }
 
-        int dimension = Environment.values().length + 1 + console.worlds.size();
+        int dimension = 10 + console.worlds.size();
         WorldServer internal = new WorldServer(console, new ServerNBTManager(new File("."), name, true), name, dimension, seed, environment, generator);
         internal.worldMaps = console.worlds.get(0).worldMaps;