Miscellaneous small improvements for scheduled_test.

pkg/scheduled_test/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 utils;
 
 import 'dart:async';
 
 /// A pair of values.
 class Pair<E, F> {
@@ skipping 74 matching lines @@
     controller.addError(e);
     controller.close();
   });
   return controller.stream;
 }
 
 // TODO(nweiz): remove this when issue 7964 is fixed.
 /// Returns a [Future] that will complete to the first element of [stream].
 /// Unlike [Stream.first], this is safe to use with single-subscription streams.
 Future streamFirst(Stream stream) {
-  // TODO(nweiz): remove this when issue 8512 is fixed.
-  var cancelled = false;
+  var stackTrace;
+  try {
+    throw '';
+  } catch (_, thrownStackTrace) {
+    stackTrace = thrownStackTrace;
+  }
+
   var completer = new Completer();
   var subscription;
   subscription = stream.listen((value) {
-    if (!cancelled) {
-      cancelled = true;
-      subscription.cancel();
-      completer.complete(value);
-    }
+    subscription.cancel();
+    completer.complete(value);
   }, onError: (e) {
-    if (!cancelled) {
-      completer.completeError(e.error, e.stackTrace);
-    }
+    completer.completeError(e.error, e.stackTrace);
   }, onDone: () {
-    if (!cancelled) {
-      completer.completeError(new StateError("No elements"));
-    }
+    completer.completeError(new StateError("No elements"), stackTrace);
   }, unsubscribeOnError: true);
   return completer.future;
 }
 
 /// A function that can be called to cancel a [Stream] and send a done message.
 typedef void StreamCanceller();
 
 // TODO(nweiz): use a StreamSubscription when issue 9026 is fixed.
 /// Returns a wrapped version of [stream] along with a function that will cancel
 /// the wrapped stream. Unlike [StreamSubscription], this canceller will send a
@@ skipping 54 matching lines @@
     return map;
   });
 }
 
 /// Returns whether [pattern] matches all of [string].
 bool fullMatch(String string, Pattern pattern) {
   var matches = pattern.allMatches(string);
   if (matches.isEmpty) return false;
   return matches.first.start == 0 && matches.first.end == string.length;
 }