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