Add testable interface for datastore.

service/datastore/raw_interface.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package datastore
 
 import (
         "fmt"
 
         "golang.org/x/net/context"
@@ skipping 174 matching lines @@
         //
         // Callback execues once per key, in the order of keys. Callback may not
         // execute at all if there's a server error.
         //
         // NOTE: Implementations and filters are guaranteed that
         //   - len(keys) > 0
         //   - all keys are Valid, !Incomplete, and in the current namespace
         //   - none keys of the keys are 'special' (use a kind prefixed with '__')
         //   - cb is not nil
         DeleteMulti(keys []Key, cb DeleteMultiCB) error
+
+        // Testable returns the Testable interface for the implementation, or nil if
+        // there is none.
+        Testable() Testable
 }