support for-await-of

[Meir017]

closes #1990

• should work for generators

esprima.parseScript('async function *g() { for await (let y of []) {} }')

• should throw an error if it's not used within an async function

esprima.parseScript('function f() { for await (let i of []) {} }')

[jsf-clabot]

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[Meir017]

@ariya, please review

[Meir017]

esprima doesn't seem to support yet parsing of async generators async function * .
I think this is out of scope of this feature and should be done in a separate PR