Make obtaining the Testable interface actually part of the interface.

service/taskqueue/taskqueue.go

Index: service/taskqueue/taskqueue.go
diff --git a/service/taskqueue/taskqueue.go b/service/taskqueue/taskqueue.go
index 97ab2ffe60d506ccac863e557b157855fb930533..5735ffb35d847f88ba5cf3b92b61865ff84e084f 100644
--- a/service/taskqueue/taskqueue.go
+++ b/service/taskqueue/taskqueue.go
@@ -75,6 +75,10 @@ func (t *taskqueueImpl) Raw() RawInterface {
 	return t.RawInterface
 }
 
+func (t *taskqueueImpl) Testable() Testable {
+	return t.RawInterface.Testable()
+}
+
 var _ Interface = (*taskqueueImpl)(nil)
 
 func Get(c context.Context) Interface {