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:     }
    }
  
    void _debugReportDuplicateIds() {
      var idSet = new HashSet<String>();
      for (var child in children) {
        if (child is Text) {
  
negative.code:   }) : this.children = children == null ? _emptyList : children,
         super(key:key) {
  
      assert(!_debugHasDuplicateIds());
    }
  
    void _remove() {
  