Async blocks for JavaScript

Eliminating IIAAFE from modules and scripts, so you don't have to.

The Idea

We've all seen the IIAAFE pattern out in the wild:

(async () => {
  // Use await in here
})();

And it's slightly more cordial cousin, the IIAMFD:

async function main() {
  // Use await in here
}

main();

We see these patterns in the following scenarios:

Documentation and education materials, where we want to demonstrate await usage but need to create an async context first.
Program "main" files, where we want to run some startup code with await in it.

Async blocks are a simple syntactic feature that allows us to avoid abstruse patterns in order to convey simple ideas.

async {
  let result = await fetch(someURL);
  let data = await result.json();
  console.log('fetched!');
}

Works in scripts and modules, too!

Here's the grammar:

Statement:
  ...
  AsyncStatement

AsyncStatement :
  `async` [no LineTerminator here] `{` StatementList[~Yield, +Await, ~Return] `}`

Note: ReturnStatement is not allowed from the statement list of an async statement.