Promotes a Build Id within an existing set to become the default ID for that set.

Class Variable	build_id	Undocumented
Method	_as_partial_proto	Returns a partial request with the operation populated. Caller must populate non-operation fields. This is done b/c there's no good way to assign a non-primitive message as the operation after initializing the request.

Returns a partial request with the operation populated. Caller must populate non-operation fields. This is done b/c there's no good way to assign a non-primitive message as the operation after initializing the request.