Use microtasks in dart.async

Use microtasks in dart.async

Not quite sure about this, but it makes ordering match for the example in  #28509

R=jmesserly@google.com

Committed: https://github.com/dart-lang/sdk/commit/a91cc9cc9e3f90e648685fb3cef92b36ab75e332

[vsm, 2017-01-25 17:49:18]

Description was changed from

==========
Await via microtask
==========

to

==========
Use microtasks in dart.async

Not quite sure about this, but it makes ordering match for the example in 
#28509
==========

[vsm, 2017-01-25 17:49:19]

vsm@google.com changed reviewers:
+ jmesserly@google.com

[Jennifer Messerly, 2017-01-25 18:19:17]

this is not correct according to the spec? See
http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-408.pdf

16.30 Await Expressions

> Otherwise, if e evaluates to an object o that is not
an instance of Future, then let f be the result of calling Future.value() with o
as
its argument; otherwise let f be the result of evaluating e.

(I don't know if there are other unpublished spec changes)

[Jennifer Messerly, 2017-01-25 18:31:18]

https://codereview.chromium.org/2653253002/diff/1/pkg/dev_compiler/tool/input...
File pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart
(right):

https://codereview.chromium.org/2653253002/diff/1/pkg/dev_compiler/tool/input...
pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart:45: var
result = future;
as noted, this change appears to contradict the spec for "await"

https://codereview.chromium.org/2653253002/diff/1/pkg/dev_compiler/tool/input...
pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart:51: return
${getGenericClass(Future)}($T).microtask(function() {

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

[Jennifer Messerly, 2017-01-25 21:46:12]

LGTM. do you want to add a test case?

[vsm, 2017-01-26 00:14:11]

Description was changed from

==========
Use microtasks in dart.async

Not quite sure about this, but it makes ordering match for the example in 
#28509
==========

to

==========
Use microtasks in dart.async

Not quite sure about this, but it makes ordering match for the example in 
#28509

R=jmesserly@google.com

Committed:
https://github.com/dart-lang/sdk/commit/a91cc9cc9e3f90e648685fb3cef92b36ab75e332
==========

[vsm, 2017-01-26 00:14:12]

Committed patchset #3 (id:40001) manually as
a91cc9cc9e3f90e648685fb3cef92b36ab75e332 (presubmit successful).