Fix most strong mode warnings.

lib/src/utils.dart

 // Copyright (c) 2014, 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 'dart:io';
 
-/// Merges all streams in [streams] into a single stream that emits all of their
-/// values.
-///
-/// The returned stream will be closed only when every stream in [streams] is
-/// closed.
-Stream mergeStreams(Iterable<Stream> streams) {
-  var subscriptions = new Set();
-  var controller;
-  controller = new StreamController(onListen: () {
-    for (var stream in streams) {
-      var subscription;
-      subscription = stream.listen(controller.add,
-          onError: controller.addError,
-          onDone: () {
-        subscriptions.remove(subscription);
-        if (subscriptions.isEmpty) controller.close();
-      });
-      subscriptions.add(subscription);
-    }
-  }, onCancel: () {
-    for (var subscription in subscriptions) {
-      subscription.cancel();
-    }
-  }, onPause: () {
-    for (var subscription in subscriptions) {
-      subscription.pause();
-    }
-  }, onResume: () {
-    for (var subscription in subscriptions) {
-      subscription.resume();
-    }
-  }, sync: true);
-
-  return controller.stream;
-}
-
 /// A cache for [supportsIpV6].
 bool _supportsIpV6;
 
 /// Returns whether this computer supports binding to IPv6 addresses.
-Future<bool> get supportsIpV6 {
-  if (_supportsIpV6 != null) return new Future.value(_supportsIpV6);
+Future<bool> get supportsIpV6 async {
+  if (_supportsIpV6 != null) return _supportsIpV6;
 
-  return ServerSocket.bind(InternetAddress.LOOPBACK_IP_V6, 0).then((socket) {
+  try {
+    var socket = await ServerSocket.bind(InternetAddress.LOOPBACK_IP_V6, 0);
     _supportsIpV6 = true;
     socket.close();
     return true;
-  }).catchError((error) {
-    if (error is! SocketException) throw error;
+  } on SocketException catch (_) {
     _supportsIpV6 = false;
     return false;
-  });
+  }
 }