tree a14fa26dccd42eb3e40cda2a93b458e23a5693c1
parent 9ec7261bd6ea1a78a3024432ee959ffe20f3e7ba
author Stephen Li <stephenli@google.com> 1490654960 -0700
committer Stephen Li <stephenli@google.com> 1491431072 +0000

Validate downstream branches exist.

Ensure that a change exists and is uploaded for each downstream
branch before allowing submit. If it does not, raise a user-facing
error so that they can correct it.

Change-Id: Ic488fd3144e31ed380879176f7a61ef330e9b0e0