This removes two bits of duplication:
  - Parsing of each AssignmentExpression, which previously was called
    first outside the loop and then inside the loop.
  - Parsing of arrow rest parameters, which previously was handled
    separately for the one-arg and N-arg cases.

The only change in behavior is in a few error messages.

Review-Url: https://codereview.chromium.org/2279363002
Cr-Commit-Position: refs/heads/master@{#39030}