anchor.diff:    String name;
     SafePropertyRead(this.receiver, this.name);
     @override
  -  dynamic visit(AstVisitor visitor, [dynamic context]) =>
  +  R visit<R, C>(AstVisitor<R, C> visitor, [C context]) =>
         visitor.visitSafePropertyRead(this, context);
   }
   
  
positive.code:   StaticRead(this.id);
  
    @override
    dynamic visit(AstVisitor visitor, [dynamic context]) =>
        visitor.visitStaticRead(this, context);
  }
  
  class ImplicitReceiver extends AST {
    @override
    dynamic visit(AstVisitor visitor, [dynamic context]) =>
        visitor.visitImplicitReceiver(this, context);
  }
  
  
negative.code:   KeyedWrite(this.obj, this.key, this.value);
  
    @override
    R visit<R, C>(AstVisitor<R, C> visitor, [C context]) =>
        visitor.visitKeyedWrite(this, context);
  }
  
  