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

pkg/unittest/lib/src/group_context.dart

 part of unittest;
 
 /// Setup and teardown functions for a group and its parents, the latter
 /// for chaining.
 class _GroupContext {
   final _GroupContext parent;
 
   /// Description text of the current test group.
   final String _name;
 
@@ skipping 36 matching lines @@
         var f = teardown();
         if (f is Future) {
           return f.then((_) => postTeardown());
         } else {
           return postTeardown();
         }
       };
     }
   }
 
-  String get fullName => (parent == null || parent == _rootContext)
+  String get fullName => (parent == null || parent == _environment.rootContext)
       ? _name
       : "${parent.fullName}$groupSep$_name";
 
   _GroupContext([this.parent, this._name = '']) {
     _testSetup = parentSetup;
     _testTeardown = parentTeardown;
   }
 }