anchor.diff:  
   final sky.Tracing _tracing = sky.window.tracing;
   
  -bool _initIsInCheckedMode() {
  -  String testFn(i) { double d = i; return d.toString(); }
  -  try {
  -    testFn('not a double');
  -  } catch (ex) {
  -    return true;
  -  }
  -  return false;
  -}
  -
  -final bool _isInCheckedMode = _initIsInCheckedMode();
   final bool _shouldLogRenderDuration = false;
   final bool _shouldTrace = false;
   
  
positive.code:   }) : this.children = children == null ? _emptyList : children,
         super(key:key) {
  
      if (_isInCheckedMode) {
        _debugReportDuplicateIds();
      }
    }
  
    void _remove() {
  
negative.code:                  of another node, they must have unique keys.''';
        }
      }
      return false;
    }
  
    void _ensureClass() {
  