Generate an error for active connections when the HTTP client is shutdown

tests/standalone/io/http_connection_close_test.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.
 //
 
 #import("dart:isolate");
 #import("dart:io");
 #import("dart:uri");
 
 void testHttp10Close(bool closeRequest) {
@@ skipping 48 matching lines @@
   };
 
   var client = new HttpClient();
   var connection =
       client.getUrl(new Uri.fromString("http://127.0.0.1:${server.port}"));
   connection.onResponse = (resp) {
     int bytes = 0;
     resp.inputStream.onData = () {
       bytes += resp.inputStream.read().length;
       if (bytes > 100) {
-        client.shutdown();
+        client.shutdown(force: true);
       }
     };
   };
+  connection.onError = (e) => Expect.isTrue(e is HttpException);
 }
 
 main() {
   testHttp10Close(false);
   testHttp10Close(true);
   testHttp11Close(false);
   testHttp11Close(true);
   testStreamResponse();
 }