anchor.diff:    ///
     /// The label does not affect the key's identity.
     const LabeledGlobalKey(this._debugLabel) : super.constructor();
  +
  +  // Used for forwarding the constructor from GlobalKey.
  +  const LabeledGlobalKey._({ String debugLabel }) : _debugLabel = debugLabel, super.constructor();
   
     final String _debugLabel;
   
  
positive.code:   ///
    /// The label is purely for debugging and not used for comparing the identity
    /// of the key.
    factory GlobalKey({ String debugLabel }) => new LabeledGlobalKey<T>(debugLabel); // the label is purely for debugging purposes and is otherwise ignored
  
    /// Creates a global key without a label.
    ///
  
negative.code:   /// Construct a [ValueKey<String>] with the given [String].
    ///
    /// This is the simplest way to create keys.
    const factory Key(String value) = ValueKey<String>;
  
    /// Default constructor, used by subclasses.
    ///
  