Removes the do-expression wrapping, modifies BytecodeGenerator change
to enter a class literal's block scope if needed.

This does not solve the actual bug in v8:6322, but helps mitigate it in
simple cases. The bug is caused by BytecodeGenerator not allocating a
large enough array of context registers to hold its entire stack,
allowing non-context registers to be overwritten during PushContext and
PopContext bytecodes.

Nevertheless, I like the idea of not depending on do-expressions when
possible, so I think it's worth doing anyways.

BUG=v8:6322
R=rmcilroy@chromium.org, marja@chromium.org, littledan@chromium.org

Change-Id: I82b7569db2a0eead1694bd04765fc4456c2f1a0a
Reviewed-on: https://chromium-review.googlesource.com/491074
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45110}