Move the desugaring into BytecodeGenerator per TODOs.

BUG=v8:6472
R=tebbi@chromium.org, rmcilroy@chromium.org, jgruber@chromium.org

Change-Id: Ic482bee18d6e6fe73de4c5f9abaf4feda7be2dd5
Reviewed-on: https://chromium-review.googlesource.com/550396Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#47403}