esnext.asynciterable.d.ts should be es2018.asynciterable.d.ts

[saschanaz]

https://github.com/Microsoft/TypeScript/blob/9706f1729e9c41c9af901c279367dec0ab1093d1/src/lib/esnext.d.ts

/// <reference lib="es2018" />
/// <reference lib="esnext.asynciterable" />
/// <reference lib="esnext.array" />
/// <reference lib="esnext.symbol" />
/// <reference lib="esnext.intl" />

Currently Symbol.asyncIterator is in esnext but it should be in es2018.

[DanielRosenwasser]

PRs welcome; just give a heads up if you decide to take a swing at this.

[saschanaz]

Opened #26220.