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:   String toString() {
      List<String> outputUnitStrings = [];
      for (var outputUnit in outputUnits) {
        List<String> importStrings = [];
        for (var import in outputUnit.imports) {
          importStrings.add(import.name);
        }
  
negative.code:     outputUnitData.deferredImportDescriptions.keys
          .forEach((ImportEntity import) {
        var importDeferName = outputUnitData.importDeferName[import];
        final fragments = fragmentsToLoad[importDeferName]!;
        final description = outputUnitData.deferredImportDescriptions[import]!;
        String getName(LibraryEntity library) {
          var name = _elementEnvironment.getLibraryName(library);
          return name == '' ? '<unnamed>' : name;
  