Make AsyncIterator comply with AsyncIterable

Similar to what is defined by Iterator Prototype.

See: https://tc39.github.io/proposal-async-iteration/#sec-asynciteratorprototype-asynciterator

packages/regenerator-runtime/runtime.js

@@ -16,6 +16,7 @@
   var undefined; // More compressible than void 0.
   var $Symbol = typeof Symbol === "function" ? Symbol : {};
   var iteratorSymbol = $Symbol.iterator || "@@iterator";
+  var asyncIteratorSymbol = $Symbol.asyncIterator || "@@asyncIterator";
   var toStringTagSymbol = $Symbol.toStringTag || "@@toStringTag";
 
   var inModule = typeof module === "object";
@@ -229,6 +230,9 @@
   }
 
   defineIteratorMethods(AsyncIterator.prototype);
+  AsyncIterator.prototype[asyncIteratorSymbol] = function () {
+    return this;
+  };
   runtime.AsyncIterator = AsyncIterator;
 
   // Note that simple async functions are implemented on top of