Support re-running tests while debugging.

lib/src/utils.dart

 // 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.
 
 library test.utils;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:math' as math;
 
@@ skipping 334 matching lines @@
 }
 
 /// Returns a [CancelableOperation] that returns the next value of [queue]
 /// unless it's canceled.
 ///
 /// If the operation is canceled, [queue] is not moved forward at all. Note that
 /// it's not safe to call further methods on [queue] until this operation has
 /// either completed or been canceled.
 CancelableOperation cancelableNext(StreamQueue queue) {
   var fork = queue.fork();
-  var completer = new CancelableCompleter(
-      onCancel: () => fork.cancel(immediate: true));
+  var canceled = false;
+  var completer = new CancelableCompleter(onCancel: () {
+    canceled = true;
+    return fork.cancel(immediate: true);
+  });
+
   completer.complete(fork.next.then((_) {
     fork.cancel();
-    return queue.next;
+    return canceled ? null : queue.next;
   }));
+
   return completer.operation;
 }
 
 /// Returns a single-subscription stream that emits the results of [operations]
 /// in the order they complete.
 ///
 /// If the subscription is canceled, any pending operations are canceled as
 /// well.
 Stream inCompletionOrder(Iterable<CancelableOperation> operations) {
   var operationSet = operations.toSet();
@@ skipping 45 matching lines @@
       urlSafe: urlSafe, addLineSeparator: addLineSeparator);
 }
 
 /// Returns middleware that nests all requests beneath the URL prefix [beneath].
 shelf.Middleware nestingMiddleware(String beneath) {
   return (handler) {
     var pathHandler = new PathHandler()..add(beneath, handler);
     return pathHandler.handler;
   };
 }