Use microtasks in dart.async

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 a88138bab1b07aaf4b95cf1014194da0bdbeb3c1..d06de04821bc13e905667643d832cb1b51c503a2 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
@@ -42,12 +42,13 @@ async_(gen, T, @rest args) => JS('', '''(() => {
     // Checks if the awaited value is a Future.
     let future = ret.value;
     if (!$instanceOf(future, ${getGenericClass(Future)})) {
-      future = $Future.value(future);
+      var result = future;

[Jennifer Messerly, 2017/01/25 18:31:17]

as noted, this change appears to contradict the spec for "await"

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

[Jennifer Messerly, 2017/01/25 18:31:17]

here's the spec for the return value, 16.14 Function Invocation:

If f is marked async (9), then a fresh instance (10.6.1) o implementing the
built-in class Future is associated with the invocation and immediately returned
to the caller. The body of f is scheduled for execution at some future time. The
future o will complete when f terminates. The value used to complete o is the
current return value (17.12), if it is defined, and the current exception (16.9)
otherwise.

... so it sounds like we should be using Completer not for "await" (spec'd to
Future.value). The correct code here would be:

    let c = ${getGenericClass(Completer)}($T);
    // ... schedule generator somehow ...
    return c.future;

the spec doesn't seem to say anything about what the scheduling should be for
this ("some future time").  If dart2js uses microtask we could certainly do a
scheduleMicrotask there. But I'd want to do our homework that this is actually
how dart2js/VM behaves... maybe look at their implementations or look at dart2js
generated code for a simple example

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