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

tests/html/xhr_test.dart

Index: tests/html/xhr_test.dart
diff --git a/tests/html/xhr_test.dart b/tests/html/xhr_test.dart
index 5759222e52866304686dbffa41b1454e8b171b91..186b596345dae32122061506dabd4caa3a4234c9 100644
--- a/tests/html/xhr_test.dart
+++ b/tests/html/xhr_test.dart
@@ -248,10 +248,11 @@ main() {
     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);
         });
     });
   });