Fix karma loader for sheriff-o-matic tests

Don't load Polymer before we include the polymer elements.

R=ojan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/517423002

git-svn-id: svn://svn.chromium.org/blink/trunk@181118 bbb929c8-8fbe-4397-9dbb-9b2b20218538