Review "running execution context" handling in AsyncBlockStart

[Ms2ger]

AsyncBlockStart does

1. Let runningContext be the running execution context.

so it can use it later in

1. Assert: When we return here, asyncContext has already been removed from the execution context stack and runningContext is the currently running execution context.

But ExecuteModule does

1. Suspend the currently running execution context.

before calling AsyncBlockStart. Does AsyncBlockStart end up with the right value?

[codehag]

This is still a TODO, will get to it this week

[codehag]

Closing per #175

[guybedford]

Reopening this as after further discussion with @codehag we determined that the current specification is self-consistent but doesn't seem to imply that microtasks should be run before asynchronous module evaluations. So this may be a spec oversight that needs more eyes on it.

[codehag]

This can now be definitively closed via tc39/ecma262#2409