Dart VM: Fix an order-of-evaluation bug in async code.

tests/language/regress_26175_test.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.
+
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+
+// Test that local variable reads and writes are sequenced correctly with
+// respect to reads and writes in an awaited Future.  See issue 26175.
+
+// Reads are sequenced correctly with respect to writes in a Future.
+Future test1() async {
+  var x = 'a';
+  f() async => x = 'b';
+  Expect.equals('abb', '${x}${await f()}${x}');
+}
+
+// Writes are sequenced correctly with respect to writes in a Future.
+Future test2(ignore) async {
+  var x;
+  f() async => x = 'b';
+  Expect.equals('abb', '${x = 'a'}${await f()}${x}');
+}
+
+// Writes are sequenced correctly with respect to reads in a Future.
+Future test3(ignore) async {
+  var x = 'a';
+  f() async => x;
+  Expect.equals('bbb', '${x = 'b'}${await f()}${x}');
+}
+
+// Repeat the same tests for static variables.
+var cell = 'a';
+
+asyncReadCell() async => cell;
+asyncWriteCell(value) async => cell = value;
+
+Future test4(ignore) async {
+  // This test assumes that it can read the initial value of cell.
+  Expect.equals('abb', '${cell}${await asyncWriteCell('b')}${cell}');
+}
+
+Future test5(ignore) async {
+  Expect.equals('abb', '${cell = 'a'}${await asyncWriteCell('b')}${cell}');
+}
+
+Future test6(ignore) async {
+  Expect.equals('bbb', '${cell = 'b'}${await asyncReadCell()}${cell}');
+}
+
+// Test that throwing is sequenced correctly with respect to other effects.
+Future test7(ignore) async {
+  cell = 'a';
+  try {
+    Expect.equals('unreachable',
+        '${throw 0}${await asyncWriteCell('b')}${cell}');
+  } catch (_) {
+    Expect.equals('a', cell);
+  }
+}
+
+main() {
+  asyncStart();
+  test1()
+      .then(test2)
+      .then(test3)
+      .then(test4)
+      .then(test5)
+      .then(test6)
+      .then(test7)
+      .then((_) => asyncEnd());
+}