anchor.diff:    /// cloning further instances.
     Expression extract(Constant constant) {
       Expression expression = constant.asExpression();
  -    replacementNodes ??= new Set<Expression>.identity();
       if (!replacementNodes.add(expression)) {
         expression = cloner.clone(expression);
         replacementNodes.add(expression);
  
positive.code: 
    InstanceBuilder instanceBuilder;
    EvaluationEnvironment env;
    Expression evaluationRoot;
    Set<TreeNode> unevaluatedNodes;
    Set<Expression> replacementNodes;
  
    int lazyDepth; // Current nesting depth of lazy regions.
  
    bool get targetingJavaScript => numberSemantics == NumberSemantics.js;
  
  
negative.code:     for (int i = 0; i < node.expressions.length; ++i) {
        entries[i] = _evaluateSubexpression(node.expressions[i]);
      }
      if (shouldBeUnevaluated) {
        final expressions = new List<Expression>(node.expressions.length);
        for (int i = 0; i < node.expressions.length; ++i) {
          expressions[i] = extract(entries[i]);
  