DDC fix for foreign futures

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
 /// stepping stone for proposed ES7 async/await, and uses ES6 Promises.
 part of dart._runtime;
 
 final _jsIterator = JS('', 'Symbol("_jsIterator")');
 final _current = JS('', 'Symbol("_current")');
 
 syncStar(gen, E, @rest args) => JS(
     '',
     '''(() => {
   const SyncIterable_E = ${getGenericClass(SyncIterable)}($E);
   return new SyncIterable_E($gen, $args);
 })()''');
 
 @JSExportName('async')
 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));
   }
   function onError(err) {
     // If the awaited Future throws, we want to convert this to an exception
     // 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) {
     let future = ret.value;
     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, $args)[Symbol.iterator]();
     var result = onValue();
     if ($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;
     }
   });
@@ skipping 165 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;
 })()''');