Update test to hopefully help detect flanky results on Windows.

tests/standalone/src/io/HttpServerEarlyClientCloseTest.dart

 #import("dart:io");
 #import("dart:isolate");
 
 void sendData(List<int> data, int port) {
   Socket socket = new Socket("127.0.0.1", port);
   socket.onConnect = () {
     socket.onData = () {
       Expect.fail("No data response was expected");
     };
     socket.outputStream.onNoPendingWrites = () {
       socket.close(false);
     };
     socket.outputStream.write(data);
   };
 }
 
 class EarlyCloseTest {
   EarlyCloseTest(Object this.data,
                  String this.exception,
                  [bool this.expectRequest = false]);
 
   Future execute(HttpServer server) {
     Completer c = new Completer();
 
     bool calledOnRequest = false;
+    bool calledOnError = false;
     server.onRequest = (HttpRequest request, HttpResponse response) {
       Expect.isTrue(expectRequest);
+      Expect.isFalse(calledOnError);
       Expect.isFalse(calledOnRequest, "onRequest called multiple times");
       calledOnRequest = true;
     };
     ReceivePort port = new ReceivePort();
     server.onError = (Exception error) {
+      Expect.isFalse(calledOnError);
       Expect.equals(exception, error.message);
-      calledOnRequest = true;  // onRequest is not allowed after onError.
+      Expect.equals(expectRequest, calledOnRequest);
+      calledOnError = true;
       port.close();
       c.complete(null);
     };
 
     List<int> d;
     if (data is List<int>) d = data;
     if (data is String) d = data.charCodes();
     if (d == null) Expect.fail("Invalid data");
     sendData(d, server.port);
 
@@ skipping 35 matching lines @@
     } else {
       server.close();
     }
   }
   runTest(tests.iterator());
 }
 
 void main() {
   testEarlyClose();
 }