Make BrowserManager resilient to an early exit.

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:math' as math;
 
 import 'package:crypto/crypto.dart';
@@ skipping 166 matching lines @@
     }
   }, onCancel: () {
     for (var subscription in subscriptions) {
       subscription.cancel();
     }
   });
 
   return controller.stream;
 }
 
+/// Returns the first value [stream] emits, or `null` if [stream] closes before
+/// emitting a value.
+Future maybeFirst(Stream stream) {
+  var completer = new Completer();
+
+  var subscription;
+  subscription = stream.listen((data) {
+    completer.complete(data);
+    subscription.cancel();
+  }, onError: (error, stackTrace) {
+    completer.completeError(error, stackTrace);
+    subscription.cancel();
+  }, onDone: () {
+    completer.complete();
+  });
+
+  return completer.future;
+}
+
 /// Returns a random base64 string containing [bytes] bytes of data.
 ///
 /// [seed] is passed to [math.Random]; [urlSafe] and [addLineSeparator] are
 /// passed to [CryptoUtils.bytesToBase64].
 String randomBase64(int bytes, {int seed, bool urlSafe: false,
     bool addLineSeparator: false}) {
   var random = new math.Random(seed);
   var data = [];
   for (var i = 0; i < bytes; i++) {
     data.add(random.nextInt(256));
@@ skipping 38 matching lines @@
   }
 }
 
 /// 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;
   };
 }