Add isEmpty

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@6686 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/main/as/com/threerings/presents/dobj/DSet.as

@@ -62,6 +62,14 @@ public class DSet
         return _entries.length;
     }
 
+    /**
+     * Returns true if this set contains no entries.
+     */
+    public function isEmpty () :Boolean
+    {
+        return _entries.length == 0;
+    }
+
    /**
      * Returns true if the set contains an entry whose
      * <code>getKey()</code> method returns a key that