anchor.diff:    static const _ReductionKind ETA_CONT = const _ReductionKind('eta-cont', 3);
     static const _ReductionKind DEAD_PARAMETER =
         const _ReductionKind('dead-parameter', 4);
  +  static const _ReductionKind BRANCH = const _ReductionKind('branch', 5);
   
     String toString() => name;
   }
  
positive.code:       worklist.add(new _ReductionTask(_ReductionKind.BETA_CONT_LIN, node));
      } else if (_isEtaCont(node)) {
        worklist.add(new _ReductionTask(_ReductionKind.ETA_CONT, node));
      }
    }
  
  
negative.code:   void processLetPrim(LetPrim node) {
      if (_isDeadVal(node)) {
        worklist.add(new _ReductionTask(_ReductionKind.DEAD_VAL, node));
      }
    }
  
    void processBranch(Branch node) {
      if (node.condition.definition is Constant) {
        worklist.add(new _ReductionTask(_ReductionKind.BRANCH, node));
      }
    }
  
  