anchor.diff:  class ExpressionBuilder
       extends GeneralizingAstVisitor /* <ast.Expression | Accessor> */ {
     final ExpressionScope scope;
  -  final ast.VariableDeclaration cascadeReceiver;
  -  ExpressionBuilder(this.scope, [this.cascadeReceiver]);
  +  ast.VariableDeclaration cascadeReceiver;
  +  ExpressionBuilder(this.scope);
   
     ast.Expression build(Expression node) {
       var result = node.accept(this);
  
positive.code: 
  class StatementBuilder extends GeneralizingAstVisitor<ast.Statement> {
    final ExpressionScope scope;
    final LabelStack breakStack, continueStack;
  
    StatementBuilder(this.scope, [this.breakStack, this.continueStack]);
  
  
negative.code: 
    ast.Statement buildInScope(
        Statement node, LabelStack breakNode, LabelStack continueNode) {
      var oldBreak = this.breakStack;
      var oldContinue = this.continueStack;
      breakStack = breakNode;
      continueStack = continueNode;
      var result = build(node);
      this.breakStack = oldBreak;
      this.continueStack = oldContinue;
      return result;
    }
  
    void buildBlockMember(Statement node, List<ast.Statement> output) {
  