Ignore synthetic classes when loading BukkitImplAdapter.

(cherry picked from commit 319f2efe43)

worldedit-bukkit/src/main/java/com/sk89q/worldedit/bukkit/adapter/BukkitImplLoader.java

@@ -93,7 +93,7 @@ public class BukkitImplLoader {
         try {
             Enumeration<JarEntry> entries = jar.entries();
             while (entries.hasMoreElements()) {
-                JarEntry jarEntry = (JarEntry) entries.nextElement();
+                JarEntry jarEntry = entries.nextElement();
 
                 String className = jarEntry.getName().replaceAll("[/\\\\]+", ".");
 
@@ -157,6 +157,7 @@ public class BukkitImplLoader {
         for (String className : adapterCandidates) {
             try {
                 Class<?> cls = Class.forName(className);
+                if (cls.isSynthetic()) continue;
                 if (BukkitImplAdapter.class.isAssignableFrom(cls)) {
                     return (BukkitImplAdapter) cls.newInstance();
                 } else {