anchor.diff:    }
   
     @override
  -  visitSetConcatenation(SetConcatenation node) {
  +  JS.Expression visitSetConcatenation(SetConcatenation node) {
       // Only occurs inside unevaluated constants.
       List<JS.Expression> entries = [];
       _concatenate(Expression node) {
  
positive.code:   }
  
    @override
    visitNot(Not node) {
      var operand = node.operand;
      if (operand is MethodInvocation && operand.name.name == '==') {
        return _emitEqualityOperator(operand.receiver, operand.interfaceTarget,
  
negative.code:   }
  
    @override
    JS.Expression visitAsExpression(AsExpression node) {
      Expression fromExpr = node.operand;
      var to = node.type;
      var jsFrom = _visitExpression(fromExpr);
  