anchor.diff:  
       Map<String, List<OutputUnit>> outputUnitsToLoad = {};
       for (var import in outputUnitData.deferredImportDescriptions.keys) {
  -      var loadId = outputUnitData.importDeferName[import];
  +      final loadId = outputUnitData.importDeferName[import]!;
         List<OutputUnit> loadList = [];
         for (var outputUnit in sortedOutputUnits) {
           assert(!outputUnit.isMainOutput);
  
positive.code: // for details. All rights reserved. Use of this source code is governed by a
  // BSD-style license that can be found in the LICENSE file.
  
  // @dart = 2.10
  
  import 'dart:collection';
  import '../../common/elements.dart' show ElementEnvironment;
  import '../../deferred_load/output_unit.dart'
      show ImportDescription, OutputUnit, OutputUnitData, deferredPartFileName;
  import '../../elements/entities.dart';
  import '../../js/js.dart' as js;
  import '../../options.dart';
  
negative.code:   final List<EmittedOutputUnit> emittedOutputUnits = [];
    final Set<PreFragment> successors = {};
    final Set<PreFragment> predecessors = {};
    late final FinalizedFragment finalizedFragment;
    int size = 0;
  
    // TODO(joshualitt): interleave dynamically when it makes sense.
    bool shouldInterleave = false;
  
  