The bytecode generator did not necessarily know for which scope, and
thus language mode, it was generating code, because it only tracked
scopes that have a context.  This led to wrong behavior in some
examples involving class expressions (which are always in strict
mode).

With this CL, the bytecode generator explicitly tracks the current
scope, independent of whether it has a context.

BUG=v8:5927

Change-Id: Ifa6b3ee5e13e07b63d00e74c7f557a328633c88b
Reviewed-on: https://chromium-review.googlesource.com/444785
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43300}