anchor.diff:  /// is an immutable description of part of a user interface. Widgets can be
   /// inflated into elements, which manage the underlying render tree.
   ///
  -/// Widgets themselves have no mutable state. If you wish to associate
  -/// mutatable state with a widget, consider using a [StatefulWidget], which
  -/// creates a [State] object (via [StatefulWidget.createState]) whenever it is
  -/// inflated into an element and incorporated into the tree.
  +/// Widgets themselves have no mutable state (all their fields must be final).
  +/// If you wish to associate mutable state with a widget, consider using a
  +/// [StatefulWidget], which creates a [State] object (via
  +/// [StatefulWidget.createState]) whenever it is inflated into an element and
  +/// incorporated into the tree.
   ///
   /// A given widget can be included in the tree zero or more times. In particular
   /// a given widget can be placed in the tree multiple times. Each time a widget
  
positive.code: ///
  /// See also:
  ///
  ///  * [State].
  ///  * [StatelessWidget].
  abstract class StatefulWidget extends Widget {
    /// Initializes [key] for subclasses.
    const StatefulWidget({ Key key }) : super(key: key);
  
negative.code: /// building a constellation of other widgets that describe the user interface
  /// more concretely. The building process continues recursively until the
  /// description of the user interface is fully concrete (e.g., consists
  /// entirely of [RenderObjectWidget]s, which describe concrete [RenderObject]s).
  ///
  /// Stateless widget are useful when the part of the user interface you are
  /// describing does not depend on anything other than the configuration
  