Stop testing IsolateChannel on browsers (#24)

Author: nex3

test/isolate_channel_test.dart

@@ -2,6 +2,8 @@
 // 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.
 
+@TestOn('vm')
+
 import 'dart:async';
 import 'dart:isolate';