This slightly simplifies scope handling. It also makes it possible to
implement some potential future changes to classes purely in the parser
by adding additional code to the DoExpression.

This is a portion of https://codereview.chromium.org/2142333002/, which
probably isn't going through in full.

Review-Url: https://codereview.chromium.org/2176653003
Cr-Commit-Position: refs/heads/master@{#38035}