[es7] Implement async functions parsing

[es7] Implement async functions parsing

Adds an implementation of "async functions" parsing according to proposal
(https://tc39.github.io/ecmascript-asyncawait/). AsyncFunctionDeclaration and
AsyncFunctionExpression are the only syntax supported by this patch, behind the
new flag --harmony-async-await.

New "async" token have been added and is parsed as a keyword only before the
"function" token, otherwise "async" is valid identifier in both strict and
non-strict modes.
Runtime %FunctionIsAsync(fn) function have been added.
Function.prototype.toString() returns "async" prefix for any async function.

BUG=v8:4483
LOG=N
R=adamk@chromium.org, rossberg@chromium.org

[rossberg, 2015-10-27 15:13:59]

Thanks for working on this patch. However, please be aware that we have a
process for adding new user-facing features to V8:

https://developers.google.com/v8/launchprocess

In particular, for substantial changes, this requires starting with design docs
that the V8 team needs to sign off on before we are going to accept patches.

I should also note that async is a particularly tricky feature that will require
deep knowledge of V8's innards. We expect this to be a multi-quarter project. In
particular, it ties in closely with generators, around which there currently are
various open questions in the V8 implementation. We have some plans for tackling
that next year, but not before we have finished ES6. To be honest, we would
prefer not to open that can right now, since ES6 already is producing a lot of
churn.

[iefserge, 2015-11-11 15:43:54]

On 2015/10/27 15:13:59, rossberg wrote:
> Thanks for working on this patch. However, please be aware that we have a
> process for adding new user-facing features to V8:
> 
> https://developers.google.com/v8/launchprocess
> 
> In particular, for substantial changes, this requires starting with design
docs
> that the V8 team needs to sign off on before we are going to accept patches.
> 
> I should also note that async is a particularly tricky feature that will
require
> deep knowledge of V8's innards. We expect this to be a multi-quarter project.
In
> particular, it ties in closely with generators, around which there currently
are
> various open questions in the V8 implementation. We have some plans for
tackling
> that next year, but not before we have finished ES6. To be honest, we would
> prefer not to open that can right now, since ES6 already is producing a lot of
> churn.

Makes sense. In case this is useful I'd be happy to rebase and make necessary
improvements to it whenever. Thank you!