Normative: handle broken promises in AsyncGenerator.prototype.return (#…

…2683)

spec.html

@@ -48562,7 +48562,14 @@ <h1>
           1. Let _next_ be the first element of _queue_.
           1. Let _completion_ be Completion(_next_.[[Completion]]).
           1. Assert: _completion_ is a return completion.
-          1. Let _promise_ be ? PromiseResolve(%Promise%, _completion_.[[Value]]).
+          1. Let _promiseCompletion_ be Completion(PromiseResolve(%Promise%, _completion_.[[Value]])).
+          1. If _promiseCompletion_ is an abrupt completion, then
+            1. Set _generator_.[[AsyncGeneratorState]] to ~completed~.
+            1. Perform AsyncGeneratorCompleteStep(_generator_, _promiseCompletion_, *true*).
+            1. Perform AsyncGeneratorDrainQueue(_generator_).
+            1. Return ~unused~.
+          1. Assert: _promiseCompletion_ is a normal completion.
+          1. Let _promise_ be _promiseCompletion_.[[Value]].
           1. Let _fulfilledClosure_ be a new Abstract Closure with parameters (_value_) that captures _generator_ and performs the following steps when called:
             1. Set _generator_.[[AsyncGeneratorState]] to ~completed~.
             1. Let _result_ be NormalCompletion(_value_).