Replace broken language/async_star_await_pauses_test

tests/language/regress_23996_test.dart

+// Copyright (c) 2016, 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 "dart:async";
+import "package:async_helper/async_helper.dart";
+
+/// This test verifies that an await for loop sends the correct
+/// signals to the stream it iterates over:
+/// 1) A listen event.
+/// 2) A pause event when the loop body is awaiting something,
+///    and more elements arrive on the stream.  See issue
+///    https://github.com/dart-lang/sdk/issues/23996 .
+/// 3) A resume event, when the loop is again ready to iterate.
+main() {
+  Completer listenEventReceived = new Completer();
+  Completer pauseEventReceived = new Completer();
+  Completer resumeEventReceived = new Completer();
+  StreamController controller = new StreamController(
+      onListen: () => listenEventReceived.complete(),
+      onPause: () => pauseEventReceived.complete(),
+      onResume: () => resumeEventReceived.complete());
+
+  Completer forLoopEntered = new Completer();
+
+  /// The send function puts items on the stream. It waits for a
+  /// listener, puts "first" on the stream, waits for the for loop
+  /// to start (and eventually block), puts "second" on the stream
+  /// multiple times, letting the event loop run, until the for loop
+  /// pauses the stream because it it blocked.
+  /// The for loop unblocks after the pause message is received, and
+  /// reads the stream items, sending a stream resume message when it
+  /// is ready for more.
+  /// Then the send function puts a final "third" on the stream, and
+  /// closes the stream.
+  send() async {
+    await listenEventReceived.future;
+    controller.add('first');
+    await forLoopEntered.future;
+    var timer = new Timer.periodic(new Duration(milliseconds: 10), (timer) {
+      controller.add('second');
+    });
+    await pauseEventReceived.future;
+    // pauseEventReceived.future completes when controller.stream is
+    // paused by the await-for loop below. What's specified is that
+    // await-for must pause immediately on an "await", but instead
+    // the implementations agree on not pausing until receiving the
+    // next event. For this reason, [timer] will call its callback at
+    // least once before we cancel it again.
+    timer.cancel();
+    await resumeEventReceived.future;
+    controller.add('third');
+    controller.close();
+  }
+
+  receive() async {
+    bool thirdReceived = false;
+    await for (var entry in controller.stream) {
+      if (entry == 'first') {
+        forLoopEntered.complete();
+        await pauseEventReceived.future;
+      } else if (entry == 'third') {
+        thirdReceived = true;
+      }
+    }
+    if (!thirdReceived) {
+      throw "Error in await-for loop: 'third' not received";
+    }
+  }
+
+  asyncTest(() async {
+    // We need to start both functions in parallel, and wait on them both.
+    var f = send();
+    await receive();
+    await f;
+  });
+}