gr-diff.html and gr-diff-root.html are shared components. Polymer modulizer doesn't convert these files automatically because they are not reachable from the application root (i.e. from gr-app.html). The gerrit-ci can fail on this change. The tests will be repaired after conversion to es6 modules. Change-Id: Icd51889a55f0ead9be953281593246dc75af9a6d