tests/html: removed usage of deprecated unittest features

tests/html/isolates_test.dart

 library IsolatesTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:async';
 import 'dart:html';
 import 'dart:convert';
 import 'dart:isolate' as isolate;
 
 String responseFor(message) => 'response for $message';
 
@@ skipping 36 matching lines @@
     port.close();
   });
   test('NonDOMIsolates', () {
     var callback = expectAsync((){});
     var response = new isolate.ReceivePort();
     var remote = isolate.Isolate.spawn(isolateEntry, response.sendPort);
     response.first.then((port) {
       final msg1 = 'foo';
       final msg2 = 'bar';
       sendReceive(port, msg1).then((response) {
-        guardAsync(() {
-          expect(response, equals(responseFor(msg1)));
-          sendReceive(port, msg2).then((response) {
-            guardAsync(() {
-              expect(response, equals(responseFor(msg2)));
-              callback();
-            });
-          });
+        expect(response, equals(responseFor(msg1)));
+        sendReceive(port, msg2).then((response) {
+          expect(response, equals(responseFor(msg2)));
+          callback();
         });
       });
     });
   });
 }