Where is the FallbackRoute attribute defined in ServiceStack?

Where is the FallbackRoute attribute defined in ServiceStack?

Here is a potentially simple question that I can seem to find the answer to:
In which namespace is the fallback route attribute defined in
ServiceStack? The wiki shows the following example, but the ServiceHost
namespace (where the Route attribute is defined) does not have a
definition for a fallback.
[FallbackRoute("/{Path}")]