Add support for the node net module.

frog/samples/node/echoServer.dart

Index: frog/samples/node/echoServer.dart
===================================================================
--- frog/samples/node/echoServer.dart	(revision 0)
+++ frog/samples/node/echoServer.dart	(revision 0)
@@ -0,0 +1,16 @@
+#library('echoServer');
+
+#import('../../lib/node/node.dart');
+#import('../../lib/node/net.dart');
+
+void main() {
+  // Note: the callback function must use the type "Socket" for the socket argument,
+  // otherwise frog will strip out the Socket.write method.
+  var server = net.createServer((Socket socket) {
+    socket.write("Echo server\r\n");
+    socket.pipe(socket);
+  });
+
+  server.listen(1337, "127.0.0.1");
+  print('Listening on port 1337');
+}