anchor.diff:      // Get a list of direct edges and then attach them to PreFragments.
       var allEdges = createDirectEdges(outputUnits);
       allEdges.forEach((outputUnit, edges) {
  -      var predecessor = outputUnitMap[outputUnit];
  +      final predecessor = outputUnitMap[outputUnit]!;
         for (var edge in edges) {
  -        var successor = outputUnitMap[edge];
  +        final successor = outputUnitMap[edge]!;
           predecessor.successors.add(successor);
           successor.predecessors.add(predecessor);
         }
  
positive.code: 
      Map<String, List<OutputUnit>> outputUnitsToLoad = {};
      for (var import in outputUnitData.deferredImportDescriptions.keys) {
        var loadId = outputUnitData.importDeferName[import];
        List<OutputUnit> loadList = [];
        for (var outputUnit in sortedOutputUnits) {
          assert(!outputUnit.isMainOutput);
  
negative.code:       List<CodeFragment> codeFragments = [];
        for (var outputUnit in outputUnits) {
          if (omittedOutputUnits.contains(outputUnit)) continue;
          final codeFragment = outputUnitMap[outputUnit]!;
          if (uniqueCodeFragments.add(codeFragment)) {
            codeFragments.add(codeFragment);
          }
          final finalizedFragment = codeFragmentMap[codeFragment]!;
          if (uniqueFinalizedFragments.add(finalizedFragment)) {
            finalizedFragments.add(finalizedFragment);
          }
  