anchor.diff:    /// Returns the [OutputUnit] where [cls] belongs.
     // TODO(johnniwinther): Remove the need for [allowNull]. Dump-info currently
     // needs it.
  -  OutputUnit outputUnitForClass(ClassEntity cls, {bool allowNull: false}) {
  +  OutputUnit outputUnitForClass(ClassEntity cls, {bool allowNull = false}) {
       if (!isProgramSplit) return mainOutputUnit;
       OutputUnit unit = _classToUnit[cls];
       assert(allowNull || unit != null, 'No output unit for class $cls');
  
positive.code: 
    CommonElements get commonElements => elementMap.commonElements;
  
    void add(ir.Expression node, {bool required: true}) {
      ConstantValue constant = elementMap
          .getConstantValue(staticTypeContext, node, requireConstant: required);
      if (constant != null) {
  
negative.code:   /// Will be `true` if the program contains deferred libraries.
    bool isProgramSplit = false;
  
    static const ImpactUseCase IMPACT_USE = ImpactUseCase('Deferred load');
  
    /// A cache of the result of calling `computeImportDeferName` on the keys of
    /// this map.
  