anchor.diff:    }
   
     FunctionBody _enclosingFunctionBody() {
  +    final parent = this.parent;
       if (parent != null) {
         return parent._enclosingFunctionBody();
       }
       var anchorExpr = virtualContainer?.widgetCreation ?? instanceCreation;
  -    return anchorExpr.thisOrAncestorOfType<FunctionBody>();
  +    return anchorExpr!.thisOrAncestorOfType<FunctionBody>()!;
     }
   
     void _formatEnclosingFunctionBody(DartFileEditBuilder builder) {
  
positive.code:           builder.write(', ');
          });
        } else {
          if (parent.virtualContainer != null) {
            parent._changeCodeVirtualContainer(builder, parameterName, buildCode);
          } else {
            parent._changeCode(builder, (builder) {
              builder.writeReference(classDescription.element);
              // TODO(scheglov) constructor name
  
negative.code:           topExpression = topArgument?.expression;
            rightExpression = rightArgument?.expression;
            bottomExpression = bottomArgument?.expression;
          } else if (constructorName == 'symmetric') {
            var hArgument = flutter.argumentByName(arguments, 'horizontal');
            var vArgument = flutter.argumentByName(arguments, 'vertical');
            leftExpression = hArgument?.expression;
  