Re-land r41544 "Update dart binaries used for running tests to version 1.7.2"

tests/html/xhr_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.
 
 library XHRTest;
 import 'dart:async';
 import 'dart:convert';
 import 'dart:html';
 import 'dart:typed_data';
 import 'package:unittest/html_individual_config.dart';
@@ skipping 230 matching lines @@
             var json = xhr.response;
             expect(json, equals(data));
           }));
     });
   });
 
   group('headers', () {
     test('xhr responseHeaders', () {
       return HttpRequest.request(url).then(
         (xhr) {
-          var serverHeader = xhr.responseHeaders['server'];
-          expect(serverHeader, isNotNull);
-          // Should be like: 'Dart/0.1 (dart:io)'
-          expect(serverHeader.startsWith('Dart/'), isTrue);
+          var contentTypeHeader = xhr.responseHeaders['content-type'];
+          expect(contentTypeHeader, isNotNull);
+          // Should be like: 'text/plain; charset=utf-8'
+          expect(contentTypeHeader.contains('text/plain'), isTrue);
+          expect(contentTypeHeader.contains('charset=utf-8'), isTrue);
         });
     });
   });
 }