It's not safe to magically compute types during representation
selection, so better disable the Typer decorator before we start
to do the representation/truncation analysis. This will also allow
us to move to a world where we can run representation selection
concurrently eventually.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2103363004
Cr-Commit-Position: refs/heads/master@{#37367}