Extendig concrete classes should no longer consume an instance id unnecessarily

- This is not tested because it would be testing too closely to the implementation

lib/class_builder.js

@@ -321,9 +321,6 @@ function createConcreteCtor( cname, members )
             return new ClassInstance();
         }
 
-        // generate and store unique instance id
-        attachInstanceId( this, ++instance_id );
-
         initInstance( this );
         this.__initProps();
 
@@ -335,6 +332,9 @@ function createConcreteCtor( cname, members )
             return;
         }
 
+        // generate and store unique instance id
+        attachInstanceId( this, ++instance_id );
+
         // call the constructor, if one was provided
         if ( this.__construct instanceof Function )
         {