Reapply "Create _nullFuture and _falseFuture in the root zone."

Originally landed by https://dart-review.googlesource.com/c/sdk/+/49509
Reverted because an internal test is fragile and changes behavior when the bug is fixed.

Change-Id: I8516082e5741547c46aa521a91826846dc101303
Reviewed-on: https://dart-review.googlesource.com/63743
Reviewed-by: Leaf Petersen <leafp@google.com>
Commit-Queue: Leaf Petersen <leafp@google.com>

CHANGELOG.md

@@ -34,6 +34,13 @@
   * Re-enable `Iterable.whereType`. The method was disabled because code
     was still being compiled in Dart 1 mode, and the function was
     error-prone when used in that code.
+* `dart:async`
+  * Changed an internal lazily-allocated reusable "null future" to always belong
+    to the root zone. This avoids race conditions where the first access to the
+    future determined which zone it would belong to. The zone is only used
+    for *scheduling* the callback of listeners, the listeners themselves will
+    run in the correct zone in any case.
+    Issue [#32556](http://dartbug.com/32556).
 
 ## 2.0.0-dev.67.0
 

sdk/lib/async/future.dart

@@ -148,10 +148,12 @@ abstract class FutureOr<T> {
  */
 abstract class Future<T> {
   /// A `Future<Null>` completed with `null`.
-  static final _Future<Null> _nullFuture = new _Future<Null>.value(null);
+  static final _Future<Null> _nullFuture =
+      new _Future<Null>.zoneValue(null, Zone.root);
 
   /// A `Future<bool>` completed with `false`.
-  static final _Future<bool> _falseFuture = new _Future<bool>.value(false);
+  static final _Future<bool> _falseFuture =
+      new _Future<bool>.zoneValue(false, Zone.root);
 
   /**
    * Creates a future containing the result of calling [computation]

sdk/lib/async/future_impl.dart

@@ -186,7 +186,7 @@ class _Future<T> implements Future<T> {
    * Until the future is completed, the field may hold the zone that
    * listener callbacks used to create this future should be run in.
    */
-  final Zone _zone = Zone.current;
+  final Zone _zone;
 
   /**
    * Either the result, a list of listeners or another future.
@@ -206,20 +206,24 @@ class _Future<T> implements Future<T> {
   var _resultOrListeners;
 
   // This constructor is used by async/await.
-  _Future();
+  _Future() : _zone = Zone.current;
 
-  _Future.immediate(FutureOr<T> result) {
+  _Future.immediate(FutureOr<T> result) : _zone = Zone.current {
     _asyncComplete(result);
   }
 
-  _Future.immediateError(var error, [StackTrace stackTrace]) {
+  /** Creates a future with the value and the specified zone. */
+  _Future.zoneValue(T value, this._zone) {
+    _setValue(value);
+  }
+
+  _Future.immediateError(var error, [StackTrace stackTrace])
+      : _zone = Zone.current {
     _asyncCompleteError(error, stackTrace);
   }
 
   /** Creates a future that is already completed with the value. */
-  _Future.value(T value) {
-    _setValue(value);
-  }
+  _Future.value(T value) : this.zoneValue(value, Zone.current);
 
   bool get _mayComplete => _state == _stateIncomplete;
   bool get _isPendingComplete => _state == _statePendingComplete;

tests/lib_2/async/null_future_zone_test.dart

@@ -0,0 +1,36 @@
+// Copyright (c) 2013, 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.
+
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+import 'dart:async';
+
+main() {
+  asyncStart(2);
+  () async {
+    var it = new StreamIterator(new Stream.fromIterable([]));
+    Expect.isFalse(await it.moveNext());
+
+    Future nullFuture;
+    Future falseFuture;
+
+    runZoned(() {
+      nullFuture = (new StreamController()..stream.listen(null).cancel()).done;
+      falseFuture = it.moveNext();
+    }, zoneSpecification: new ZoneSpecification(scheduleMicrotask:
+        (Zone self, ZoneDelegate parent, Zone zone, void f()) {
+      Expect.fail("Should not be called");
+    }));
+
+    nullFuture.then((value) {
+      Expect.isNull(value);
+      asyncEnd();
+    });
+
+    falseFuture.then((value) {
+      Expect.isFalse(value);
+      asyncEnd();
+    });
+  }();
+}