Yield in async generator should implicitly unwrap operand

[rbuckton]

This fixes an issue where we do not implicitly await the operand to yield or the iterated elements of the operand to yield* as per https://tc39.github.io/proposal-async-iteration/#sec-asyncgenerator-definitions-evaluation.

Fixes #15205

[rbuckton]

I will also need to patch tslib.

[ComFreek]

I can still reproduce this issue with TypeScript 2.9.0-dev.20180422 on Windows 10 Pro 64-bit.

// Run with "tsc && node --harmony test.js"
// Node.js version: 9.7.1

declare const console: any;

async function *asyncGenerator(): AsyncIterable<number> {
  yield 1;
  yield Promise.resolve(2);
}

(async function() {
	const iterator: AsyncIterator<number> = asyncGenerator()[Symbol.asyncIterator]();

	// Should be {value: 1, done: false}
	console.log(await iterator.next());

	// Should be {value: 2, done: false}
	//
	// But actually is { value: Promise { 2 }, done: false } with the
	// following targets: es5, es6, es20{15, 16, 17, 18}.
	// The target "esnext" prevents any downcompilation and therefore
	// works natively on Node.js/V8 as intended.
	console.log(await iterator.next());

	// Should be {value: undefined, done: true}
	console.log(await iterator.next());
})();

tsconfig.json:

{
  "compilerOptions": {
    "target": "es5",
    "module": "commonjs",
    "lib": [
      "es6",
      "esnext.asynciterable",
    ],

    "importHelpers": true,
    "downlevelIteration": true,

    "strict": true,
  }
}