Extract callback validation into method.

laravel/routing/route.php

@@ -61,12 +61,25 @@ public function __construct($key, $callback, $parameters = array())
 			$this->uris = array_map(array($this, 'extract'), explode(', ', $key));
 		}
 
-		if ( ! $callback instanceof Closure and ! is_array($callback) and ! is_string($callback))
+		if ( ! $this->callable($callback))
 		{
 			throw new \InvalidArgumentException('Invalid route defined for URI ['.$this->key.']');
 		}
 	}
 
+	/**
+	 * Determine if the given route callback is callable.
+	 *
+	 * Route callbacks must be either a Closure, array, or string.
+	 *
+	 * @param  mixed  $callback
+	 * @return bool
+	 */
+	protected function callable($callback)
+	{
+		return $callback instanceof Closure or is_array($callback) or is_string($callback);
+	}
+
 	/**
 	 * Retrieve the URI from a given route destination.
 	 *