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: /// The filename is of the form "<main output file>_<name>.part.js".
  /// If [addExtension] is false, the ".part.js" suffix is left out.
  String deferredPartFileName(CompilerOptions options, String name,
      {bool addExtension = true}) {
    assert(name != "");
    String outPath = options.outputUri != null ? options.outputUri.path : "out";
    String outName = outPath.substring(outPath.lastIndexOf('/') + 1);
  