After #9626 is merged, we will no longer be doing a full gulp dist for unit tests, and instead, will be doing a gulp build followed by a gulp test --nobuild. When I tested this change, I noticed that the tests in amp-ad-network-doubleclick-impl fail. It turns out this is due to a missing stub.

See https://travis-ci.org/ampproject/amphtml/jobs/237958534 for full failure logs