lower async generator functions

[evanw]

With this PR, esbuild will now transform async generator functions into normal generator functions when the configured target environment doesn't support them. These functions behave similar to normal generator functions except that they use the Symbol.asyncIterator interface instead of the Symbol.iterator interface and the iteration methods return promises. Here's an example (helper functions are omitted):

// Original code
async function* foo() {
  yield Promise.resolve(1)
  await new Promise(r => setTimeout(r, 100))
  yield *[Promise.resolve(2)]
}
async function bar() {
  for await (const x of foo()) {
    console.log(x)
  }
}
bar()

// New output (with --target=es6)
function foo() {
  return __asyncGenerator(this, null, function* () {
    yield Promise.resolve(1);
    yield new __await(new Promise((r) => setTimeout(r, 100)));
    yield* __yieldStar([Promise.resolve(2)]);
  });
}
function bar() {
  return __async(this, null, function* () {
    try {
      for (var iter = __forAwait(foo()), more, temp, error; more = !(temp = yield iter.next()).done; more = false) {
        const x = temp.value;
        console.log(x);
      }
    } catch (temp) {
      error = [temp];
    } finally {
      try {
        more && (temp = iter.return) && (yield temp.call(iter));
      } finally {
        if (error)
          throw error[0];
      }
    }
  });
}
bar();

This is an older feature that was added to JavaScript in ES2018 but I didn't implement the transformation then because it's a rarely-used feature. Note that esbuild already added support for transforming for await loops (the other part of the asynchronous iteration proposal) a year ago, so support for asynchronous iteration should now be complete.

I have never used this feature myself and code that uses this feature is hard to come by, so this transformation has not yet been tested on real-world code. If you do write code that uses this feature, please let me know if esbuild's async generator transformation doesn't work with your code.

Fixes #2780

[fedellen]

Thanks for the fix and the write up here ❤️

[tido64]

@evanw: Hi, I'm currently debugging a crash we're seeing after bumping esbuild to a more recent version where the only diff is the addition of this polyfill. Is this line meant to throw if asyncIterator is not defined/supported by the VM? What is then the reason for the check on line 390 below?

Edit: Would it make sense to switch the order so that we check iterator first then asyncIterator, or is that simply working around the issue?