Some scopes are introduced by the parser for desugaring and do not
have any positions associated. The debugger should not make them
visible.

Also add some missing source positions.

R=kozyatinskiy@chromium.org, rossberg@chromium.org
BUG=chromium:604458
LOG=Y

Review URL: https://codereview.chromium.org/1901413002

Cr-Commit-Position: refs/heads/master@{#35721}