Fix async iterator runtime type errors

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

Index: pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart
index b90e2ccc3b8ca0884c8df91d168d968c51464487..84950793516294a15bd08650bf3de2e05a41980d 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart
@@ -25,6 +25,9 @@ syncStar(gen, E, @rest args) => JS(
 async_(gen, T, @rest args) => JS(
     '',
     '''(() => {
+  let FutureRaw = ${getGenericClass(Future)};
+  let FutureT = FutureRaw($T);
+  let FutureD = FutureRaw(dart.dynamic);
   let iter;
   function onValue(res) {
     if (res === void 0) res = null;
@@ -45,13 +48,13 @@ async_(gen, T, @rest args) => JS(
     if (ret.done) return ret.value;
     // Checks if the awaited value is a Future.
     let future = ret.value;
-    if (!$instanceOf(future, ${getGenericClass(Future)})) {
-      future = $Future.value(future);
+    if (!$instanceOf(future, FutureD)) {
+      future = FutureT.value(future);

[Jennifer Messerly, 2017/03/23 00:31:19]

Chatted w/ Vijay offline ... I don't think this is right, because this is legal
in Dart:

Future<int> f() async {
  var a = await 'hello';
  var b = await 'world'
  return (a + b).length; // 10
}


... so we need to have the final Future be typed Future<T>, while allowing the
intermediate ones to be Future<top type> ... it may be that we can't use our
implementation like this anymore and we need to track the "final" one separately
with a Completer. IIRC that's how dart2js implements it.

     }
     // Chain the Future so `await` receives the Future's value.
-    return future.then($dynamic)(onValue, {onError: onError});
+    return future.then($T)(onValue, {onError: onError});
   }
-  return ${getGenericClass(Future)}($T).microtask(function() {
+  return FutureT.microtask(function() {
     iter = $gen.apply(null, $args)[Symbol.iterator]();
     return onValue();
   });