The old proxy only mocked out constructor calls and didn't intercept function application. It also kept the original constructor property, through which non-mocked dates could be constructed again.

BUG=chromium:697870
NOTRY=true
R=mstarzinger@chromium.org,yangguo@chromium.org

Change-Id: Icb4ef22342424f95463a7a9c57fa0bb8d910ac19
Reviewed-on: https://chromium-review.googlesource.com/448564Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43569}