DDC fix for foreign futures

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 2d6dbb9156673e6b413e796bbad74b4d1d59574c..bc5a719eb6b8fbac670e64ed9fd08828e033b096 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
@@ -26,6 +26,8 @@ async_(gen, T, @rest args) => JS(
     '',
     '''(() => {
   let iter;
+  let FutureT = ${getGenericClass(Future)}($T);

[Jennifer Messerly, 2017/04/06 20:01:52]

const FutureT = ... ?

[vsm, 2017/04/06 21:31:32]

Done.

+  let _Future = $Future.new(() => null).runtimeType;  

[vsm, 2017/04/06 19:45:05]

Is there a better way to get this type?

[Jennifer Messerly, 2017/04/06 20:01:52]

good question. I'd do it like this:

    let _FutureType;

... more notes below.

[vsm, 2017/04/06 21:31:32]

Acknowledged.

   function onValue(res) {
     if (res === void 0) res = null;
     return next(iter.next(res));
@@ -46,14 +48,13 @@ async_(gen, T, @rest args) => JS(
     if (ret.done) {
       return ret.value;
     }
-    // Checks if the awaited value is a Future.
-    if (!$instanceOf(future, $Future)) {
+    // Wraps if future is not a native Future.
+    if (!$instanceOf(future, _Future)) {

[Jennifer Messerly, 2017/04/06 20:01:52]

you could just match the exact runtime type here, rather than an "is" check?

    if ($_getRuntimeType(future) !== _FutureType) {

... should be a bit faster.

[vsm, 2017/04/06 21:31:32]

I don't think that's quite right - _Future<int> != _Future<dynamic> but doesn't
need to be wrapped.
But, modified to work with getGenericClass - that works.

       future = $Future.value(future);
     }
     // Chain the Future so `await` receives the Future's value.
     return future.then($dynamic)(onValue, {onError: onError});
   }
-  let FutureT = ${getGenericClass(Future)(T)};
   return FutureT.microtask(function() {

[Jennifer Messerly, 2017/04/06 20:01:52]

    let result = FutureT.microtask(function() { ... });
    // TODO(jmesserly): optimize this further.
    _FutureType = ${_getRuntimeType(getGenericClass(result)}();
    return result;

Basically, it's similar to your approach but avoids allocating another Future.
We can get the runtime type from the one we already have by hopping to
_Future<T> and then applying <dynamic>.

... we really should fix this though. We need a way to get the library objects
reliably.

A simpler alternative is:

    const _FutureType = async._Future;

and just rely on our naming scheme? Seems okay to me but I'm not sure if it
works.

[vsm, 2017/04/06 21:31:32]

Clever!  Done with some slight mods.

     iter = $gen.apply(null, $args)[Symbol.iterator]();
     var result = onValue();