Fix async iterator runtime type errors

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// This library adapts ES6 generators to implement Dart's async/await.
 /// It's designed to interact with Dart's Future/Stream and follow Dart
 /// async/await semantics.
 /// See https://github.com/dart-lang/sdk/issues/27315 for ideas on
 /// reconciling Dart's Future and ES6 Promise.
 /// Inspired by `co`: https://github.com/tj/co/blob/master/index.js, which is a
@@ skipping 24 matching lines @@
     // thrown from the `yield` point, as if it was thrown there.
     //
     // If the exception is not caught inside `gen`, it will emerge here, which
     // will send it to anyone listening on this async function's Future<T>.
     //
     // In essence, we are giving the code inside the generator a chance to
     // use try-catch-finally.
     return next(iter.throw(err));
   }
   function next(ret) {
-    if (ret.done) return ret.value;
+    let future = ret.value;
+    if (ret.done) {
+      return ret.value;
+    }
     // Checks if the awaited value is a Future.
-    let future = ret.value;
-    if (!$instanceOf(future, ${getGenericClass(Future)})) {
+    if (!$instanceOf(future, $Future)) {
       future = $Future.value(future);
     }
     // Chain the Future so `await` receives the Future's value.
     return future.then($dynamic)(onValue, {onError: onError});
   }
-  return ${getGenericClass(Future)}($T).microtask(function() {
+  let FutureT = ${getGenericClass(Future)(T)};
+  return FutureT.microtask(function() {
     iter = $gen.apply(null, $args)[Symbol.iterator]();
-    return onValue();
+    var result = onValue();
+    if ($strongInstanceOf(result, FutureT) == null) {
+      // Chain the Future<dynamic> to a Future<T> to produce the correct
+      // final type.
+      return result.then($T)((x) => x, {onError: onError});
+    } else {
+      return result;
+    }
   });
 })()''');
 
 // Implementation inspired by _AsyncStarStreamController in
 // dart-lang/sdk's runtime/lib/core_patch.dart
 //
 // Given input like:
 //
 //     foo() async* {
 //       yield 1;
@@ skipping 156 matching lines @@
   }
 ''');
 
 /// Returns a Stream of T implemented by an async* function. */
 ///
 asyncStar(gen, T, @rest args) => JS(
     '',
     '''(() => {
   return new $_AsyncStarStreamController($gen, $T, $args).controller.stream;
 })()''');