Although the `for..in` statement allows Expressions to define the
iterator, only an AssignmentExpression may occupy this position in the
`for..of` statement.

BUG=v8:4692
LOG=N
R=adamk@chromium.org

Review URL: https://codereview.chromium.org/1602823003

Cr-Commit-Position: refs/heads/master@{#33420}