anchor.diff:    void flushSemantics() {
       if (_semanticsOwner == null)
         return;
  -    Timeline.startSync('Semantics');
  +    profile(() { Timeline.startSync('Semantics'); });
       assert(_semanticsOwner != null);
       assert(() { _debugDoingSemantics = true; return true; }());
       try {
  
positive.code:         }
        }
      } finally {
        _debugDoingLayout = false;
        Timeline.finishSync();
      }
    }
  
  
negative.code:     } finally {
        assert(_nodesNeedingSemantics.isEmpty);
        assert(() { _debugDoingSemantics = false; return true; }());
        profile(() { Timeline.finishSync(); });
      }
    }
  }
  