iterator: unbreak pre-1.7 builds
iterator: unbreak pre-1.7 builds

Change-Id: I3767f9240c350987f72da0d095207d79becb44db
Reviewed-on: https://code-review.googlesource.com/7090
Reviewed-by: Dave Day <4cba7a21e1f1fc896442779dcb55aeda6df97dfe@golang.org>

google/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client,googleapis/google-api-go-client