Support passing an environment variable map to child processes.

tests/standalone/src/io/TestingServer.dart

 // Copyright (c) 2012, 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 onConnection();
+  abstract void onConnection(Socket connection);
 
   void main() {
     void errorHandlerServer(Exception e) {
       Expect.fail("Server socket error $e");
     }
 
     this.port.receive((message, SendPort replyTo) {
       if (message == INIT) {
         _server = new ServerSocket(HOST, 0, 10);
         Expect.equals(true, _server !== null);
@@ skipping 33 matching lines @@
 
   void shutdown() {
     _sendPort.send(TestingServer.SHUTDOWN, _receivePort.toSendPort());
     _receivePort.close();
   }
 
   int _port;
   ReceivePort _receivePort;
   SendPort _sendPort;
 }