Fix bugs in HttpBody detected by the analyzer

tests/standalone/io/http_body_test.dart

 // Copyright (c) 2013, 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:io';
 import 'dart:utf';
 
 import 'package:expect/expect.dart';
 
 void testHttpClientResponseBody() {
@@ skipping 14 matching lines @@
             });
       });
 
       var client = new HttpClient();
       client.get("127.0.0.1", server.port, "/")
           .then((request) => request.close())
           .then(HttpBodyHandler.processResponse)
           .then((body) {
             if (shouldFail) Expect.fail("Error expected");
             Expect.equals(type, body.type);
+            Expect.isNotNull(body.response);
             switch (type) {
               case "text":
                 Expect.equals(expectedBody, body.body);
                 break;
 
               case "json":
                 Expect.mapEquals(expectedBody, body.body);
                 break;
 
               default:
@@ skipping 116 matching lines @@
        "json",
        true);
 
   test(null, "body".codeUnits, "body".codeUnits, "binary");
 }
 
 void main() {
   testHttpClientResponseBody();
   testHttpServerRequestBody();
 }