anchor.diff:    /// Rewriter for nested functions.
     StatementRewriter.nested(StatementRewriter parent)
         : constantEnvironment = parent.constantEnvironment,
  -        seenUses = parent.seenUses,
  +        unseenUses = parent.unseenUses,
           isDartMode = parent.isDartMode;
   
     /// A set of labels that can be safely inlined at their use.
  
positive.code:       if (getLeftHand(binding) == node.variable && isFirstUse) {
          environment.removeLast();
          --node.variable.readCount;
          --seenUses[node.variable]; // Do not count the use we just destroyed.
          return visitExpression(binding);
        }
  
  
negative.code:   /// for a break to L' if L maps to L'.
    Map<Label, Jump> labelRedirects = <Label, Jump>{};
  
    /// Number of uses of the given variable that are still unseen.
    /// Used to detect the first use of a variable (since we do backwards
    /// traversal, the first use is the last one seen).
    Map<Variable, int> unseenUses = <Variable, int>{};
  
    /// Rewriter for methods.
    StatementRewriter({this.isDartMode})
  