Refactor some of the duplicated code in the tests using a socket server

tests/standalone/src/TestingServer.dart

+// Copyright (c) 2011, 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.
+
+class TestingServer extends Isolate {
+
+  static final HOST = "127.0.0.1";
+  static final INIT = 0;
+  static final SHUTDOWN = -1;
+
+  abstract void connectionHandler();
+
+  void main() {
+    void errorHandlerServer() {
+      Expect.fail("Server socket error");
+    }
+
+    this.port.receive((message, SendPort replyTo) {
+      if (message == INIT) {
+        _server = new ServerSocket(HOST, 0, 10);
+        Expect.equals(true, _server !== null);
+        _server.connectionHandler = connectionHandler;
+        _server.errorHandler = errorHandlerServer;
+        replyTo.send(_server.port, null);
+      } else if (message == SHUTDOWN) {
+        _server.close();
+        this.port.close();
+      }
+    });
+  }
+
+  ServerSocket _server;
+}