anchor.diff:            _asCascadeSection(node.leftHandSide), node.operator.type,
             node.rightHandSide);
       }
  +    return unreachable(node);
     }
   
     visitCascadeExpression(ast.CascadeExpression node) => (f, s) {
  
positive.code: 
    bool visitParenthesizedExpression(ast.ParenthesizedExpression node) {
      return maybeAdd(node, visit(node.expression));
    }
  
    bool visitPrefixedIdentifier(ast.PrefixedIdentifier node) {
  
negative.code:     return visit(node.expression)(f, s);
    };
  
    visitPostfixExpression(ast.PostfixExpression node) => (f, s) {
      return visit(node.operand)(f, (expr) {
        return s(AstFactory.postfixExpression(expr, node.operator.type));
      });
    };
  
    visitPrefixExpression(ast.PrefixExpression node) => (f, s) {
      return visit(node.operand)(f, (expr) {
  