Refactor the close and error handling of HTTP connections

tests/standalone/src/io/HttpServerEarlyServerCloseTest.dart

 #import("dart:io");
 #import("dart:isolate");
 
 class Server {
   Server() {
     HttpServer server = new HttpServer();
     server.listen("127.0.0.1", 0);
     port = server.port;
     server.onRequest = (HttpRequest request, HttpResponse response) {
-      new Timer(100, (timer) => server.close());
+      new Timer(0, (timer) => server.close());
     };
-    server.onError = (Object exception) {
-      Expect.fail("Close should not give an error.");
+    server.onError = (e) {
+      Expect.fail("No server errors expected: $e");
     };
   }
   int port;
 }
 
 class Client {
   Client(int port) {
     ReceivePort r = new ReceivePort();
     HttpClient client = new HttpClient();
     HttpClientConnection c = client.get("127.0.0.1", port, "/");
     c.onRequest = (HttpClientRequest request) {
       request.outputStream.close();
     };
     c.onResponse = (HttpClientResponse response) {
       Expect.fail("Response should not be given, as not data was returned.");
     };
-    c.onError = (Object exception) {
+    c.onError = (e) {
       r.close();
     };
   }
 }
 
 main() {
   Server server = new Server();
   new Client(server.port);
 }