anchor.diff:          (node.parent as AssignmentExpression).leftHandSide == node &&
           node.parent.parent is ExpressionStatement) {
       } else {
  -      _coverageMarker();
         return;
       }
       var assignExpression = node.parent as AssignmentExpression;
       // check that binary expression is assignment
       if (assignExpression.operator.type != TokenType.EQ) {
  -      _coverageMarker();
         return;
       }
       // prepare "declaration" statement
       var element = (node as SimpleIdentifier).staticElement;
       if (element == null) {
  -      _coverageMarker();
         return;
       }
       var declOffset = element.nameOffset;
  
positive.code:   Future<void> _addProposal_flutterSwapWithChild() async {
      var parent = flutter.identifyNewExpression(node);
      if (!flutter.isWidgetCreation(parent)) {
        _coverageMarker();
        return;
      }
  
      var childArgument = flutter.findChildArgument(parent);
      if (childArgument?.expression is! InstanceCreationExpression ||
          !flutter.isWidgetCreation(childArgument.expression)) {
        _coverageMarker();
        return;
      }
      InstanceCreationExpression child = childArgument.expression;
  
negative.code:     }
      // prepare IsExpression
      if (node is! IsExpression) {
        return;
      }
      IsExpression isExpression = node;
  