deps: fix async await desugaring in V8

[targos]

This is a backport of https://codereview.chromium.org/2672313003/. The
patch did not land in V8 because it was superseded by another one but it
is much easier to backport to V8 5.5, was reviewed and passed tests.

Original commit message:

[async await] Fix async function desugaring
Previously we rewrote the return statement in an async function from
`return expr;` to `return %ResolvePromise(.promise, expr),
.promise`. This can lead to incorrect behavior in the presence of try-finally.
This patch stores the `expr` of the return statement in a temporary
variable, resolves and returns the promise at the end of the finally
block.
BUG=v8:5896

Fixes: #11960

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows [commit guidelines][]

Affected core subsystem(s)

V8

[targos]

CI: https://ci.nodejs.org/job/node-test-pull-request/6997/
V8 CI: https://ci.nodejs.org/job/node-test-commit-v8-linux/620/

/cc @nodejs/v8

[targos]

There seems to be a problem with Clang: https://ci.nodejs.org/job/node-test-commit-linux/8639/nodes=ubuntu1204-clang341-64/console

[bnoordhuis]

@targos It's not just you, #11652 (comment).

[jasnell]

@nodejs/build ^^^

[bnoordhuis]

LGTM with a nit and a question.

[bnoordhuis]

Long line.

[targos]

fixed

[bnoordhuis]

I realize the original CL does that too but do you happen to know why it uses empty_string() instead of undefined_value() or the_hole_value()?

[targos]

I agree, there doesn't seem to be a good reason for that. Do you want me to change it for undefined_value()?

[targos]

@gsathya ^

[bnoordhuis]

I think the logic is that the initial value won't matter because it's overwritten in all code paths? I'd have used the_hole_value() because its presence traps in many cases and it's something you can unambiguously debug-check since it's never produced by JS code.

[gsathya]

empty_string isn't the value, it's the name of the variable. we never actually lookup the variable by name, so it doesn't need a name.