Use microtasks in dart.async

pkg/dev_compiler/lib/js/amd/dart_sdk.js

Index: pkg/dev_compiler/lib/js/amd/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/amd/dart_sdk.js b/pkg/dev_compiler/lib/js/amd/dart_sdk.js
index 2b78591de33500ddffa71e811a2709f6a2fba70c..91f2480a00ce8b188fc309788bff077951db2185 100644
--- a/pkg/dev_compiler/lib/js/amd/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/amd/dart_sdk.js
@@ -1864,11 +1864,12 @@ define([], function() {
       if (ret.done) return ret.value;
       let future = ret.value;
       if (!dart.is(future, dart.getGenericClass(async.Future))) {
-        future = async.Future.value(future);
+        var result = future;
+        future = async.Future.microtask(() => result);
       }
       return future.then(dart.dynamic)(onValue, {onError: onError});
     }
-    return dart.getGenericClass(async.Future)(T).new(function() {
+    return dart.getGenericClass(async.Future)(T).microtask(function() {
       iter = gen.apply(null, args)[Symbol.iterator]();
       return onValue();
     });