Add reusePort argument to ServerSocket.

tests/standalone/io/socket_reuse_port_test.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.
+
+import "dart:async";
+import "dart:io";
+
+
+void testReuseSocket(int numConnections, int numServers) {
+  ServerSocket.bind('localhost', 0, reusePort: true).then((server) {
+    var servers = [];
+    for (int i = 1; i < numServers; i++) {
+      servers.add(ServerSocket.bind('localhost', server.port, reusePort: true));
+    }
+    Future.wait(servers).then((servers) {
+      servers = [server]..addAll(servers);
+      for (var server in servers) {
+        server.listen((socket) {
+          socket.pipe(socket);
+        });
+      }
+      var connections = [];
+      for (int i = 0; i < numConnections; i++) {
+        connections.add(
+            Socket.connect('localhost', server.port)
+                .then((socket) => socket.close())
+                .then((socket) => socket.listen(null).asFuture()));
+      }
+      Future.wait(connections).then((_) {
+        for (var server in servers) {
+          server.close();
+        }
+      });
+    });
+  }).catchError((e) {
+    print("reusePort is not supported: $e");
+  });
+}
+
+void main() {
+  testReuseSocket(1, 1);
+  testReuseSocket(10, 1);
+  testReuseSocket(10, 10);
+  testReuseSocket(100, 10);
+}