Enhance the Dart unittest library to support multiple invocations in the same application run.

pkg/unittest/lib/src/group_context.dart

Index: pkg/unittest/lib/src/group_context.dart
diff --git a/pkg/unittest/lib/src/group_context.dart b/pkg/unittest/lib/src/group_context.dart
index 49c83a4681371d4782a8b03107553f131df52601..d44b6cd3fec4a21a7c82ab2795b99fa5cd06e5ce 100644
--- a/pkg/unittest/lib/src/group_context.dart
+++ b/pkg/unittest/lib/src/group_context.dart
@@ -54,7 +54,7 @@ class _GroupContext {
     }
   }
 
-  String get fullName => (parent == null || parent == _rootContext)
+  String get fullName => (parent == null || parent == _environment.rootContext)
       ? _name
       : "${parent.fullName}$groupSep$_name";