Support native async functions (node 7.6+)

[alexeyraspopov]

Long story short:

~
λ node --version
v7.7.1
~
λ node
> Object.prototype.toString.call(async function(){})
'[object AsyncFunction]'

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[alexeyraspopov]

I signed it!

[googlebot]

CLAs look good, thanks!

[alexeyraspopov]

@sjelin, could you please review?

[alexeyraspopov]

Anyone from Protractor team can review this please? @juliemr@cnishina

[alexeyraspopov]

Worth mentioning, this is urgent since WebDriver's promise manager will be deprecated soon:
https://github.com/SeleniumHQ/selenium/wiki/WebDriverJs#moving-to-asyncawait

[arielserafini]

+1 for this. Any updates?

[heathkit]

Thanks for the fix!

[renehamburger]

Thanks for this! Would be great to add a new release to get this update into protractor.