'Replace the existing stub for the specified service with a new one.
NOTE: This is a risky operation; external callers should use this with
caution.
Args:
service: string
stub: stub'

def ReplaceStub(self, service, stub):

    self.__stub_map[service] = stub
    if (service == 'datastore'):
        self.RegisterStub('datastore_v3', stub)
