anchor.diff:      bool softWrap,
       TextOverflow overflow,
       int maxLines,
  -    Widget child
  +    @required Widget child,
     }) {
       assert(context != null);
       assert(child != null);
  
positive.code:   /// Creates a default text style for the given subtree.
    ///
    /// Consider using [DefaultTextStyle.merge] to inherit styling information
    /// from a the current default text style for a given [BuildContext].
    DefaultTextStyle({
      Key key,
      @required this.style,
  
negative.code:   /// A const-constructible default text style that provides fallback values.
    ///
    /// Returned from [of] when the given [BuildContext] doesn't have an enclosing default text style.
    ///
    /// This constructor creates a [DefaultTextStyle] that lacks a [child], which
    /// means the constructed value cannot be incorporated into the tree.
    const DefaultTextStyle.fallback()
      : style = const TextStyle(),
        textAlign = null,
  