lib/Twig/lib/Twig/Environment.php
public function getRuntime($class)
    {
        if (isset($this->runtimes[$class])) {
            return $this->runtimes[$class];
        }

        foreach ($this->runtimeLoaders as $loader) {
            if (null !== $runtime = $loader->load($class)) {
                return $this->runtimes[$class] = $runtime;
            }
        }

        throw new Twig_Error_Runtime(sprintf('Unable to load the "%s" runtime.', $class));
    }
public function getRuntime($class)
    {
        if (isset($this->runtimes[$class])) {
            return $this->runtimes[$class];
        }

        foreach ($this->runtimeLoaders as $loader) {
            if (null !== $runtime = $loader->load($class)) {
                return $this->runtimes[$class] = $runtime;
            }
        }

        throw new Twig_Error_Runtime(sprintf('Unable to load the "%s" runtime.', $class));
    }
Returns the runtime implementation of a Twig element (filter/function/test).

@param string $class A runtime class name

@return object The runtime implementation

@throws Twig_Error_Runtime When the template cannot be found