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: 
    /// Returns the [OutputUnit] where [cls]'s type belongs.
    // TODO(joshualitt): see above TODO regarding allowNull.
    OutputUnit outputUnitForClassType(ClassEntity cls, {bool allowNull: false}) {
      if (!isProgramSplit) return mainOutputUnit;
      OutputUnit unit = _classTypeToUnit[cls];
      assert(allowNull || unit != null, 'No output unit for type $cls');
  
negative.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) {
  