anchor.diff:    /// Construct a [ValueKey<String>] with the given [String].
     ///
     /// This is the simplest way to create keys.
  -  factory Key(String value) => new ValueKey<String>(value);
  +  const factory Key(String value) = ValueKey<String>;
   
     /// Default constructor, used by subclasses.
     ///
  
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:   ///
    /// 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;
  
  