This became temporarily a big issue, because spreads are desugared
into do-expressions.  This patch fixes the problem with having
spreads as parameter initializers in arrow expressions, e.g., this
line would crash:

    [], ((x = [...[42]]) => x)();

R=rossberg@chromium.org
BUG=chromium:578038
LOG=N

Review URL: https://codereview.chromium.org/1581403007

Cr-Commit-Position: refs/heads/master@{#33365}