anchor.diff:      // sdkSummary between multiple invocations.
       CanonicalName nameRoot = sdkSummary?.root ?? new CanonicalName.root();
       if (sdkSummary != null) {
  +      if (options.nnbdMode == NnbdMode.Strong &&
  +          !(sdkSummary.mode == NonNullableByDefaultCompiledMode.Strong ||
  +              sdkSummary.mode == NonNullableByDefaultCompiledMode.Agnostic)) {
  +        throw new FormatException(
  +            'Provided SDK .dill does not support sound null safety.');
  +      }
         dillTarget.loader.appendLibraries(sdkSummary);
       }
   
       for (Component additionalDill
           in await options.loadAdditionalDills(nameRoot)) {
  +      if (options.nnbdMode == NnbdMode.Strong &&
  +          !(additionalDill.mode == NonNullableByDefaultCompiledMode.Strong ||
  +              // In some VM tests the SDK dill appears as an additionalDill so
  +              // allow agnostic here as well.
  +              additionalDill.mode ==
  +                  NonNullableByDefaultCompiledMode.Agnostic)) {
  +        throw new FormatException(
  +            'Provided .dill file for the following libraries does not support '
  +            'sound null safety:\n'
  +            '${additionalDill.libraries.join('\n')}');
  +      }
         loadedComponents.add(additionalDill);
         dillTarget.loader.appendLibraries(additionalDill);
       }
  
positive.code:                 : summaryComponent.libraries);
        trimmedSummaryComponent.metadata.addAll(summaryComponent.metadata);
        trimmedSummaryComponent.uriToSource.addAll(summaryComponent.uriToSource);
  
        // As documented, we only run outline transformations when we are building
        // summaries without building a full component (at this time, that's
  
negative.code: 
  import 'package:_fe_analyzer_shared/src/messages/severity.dart' show Severity;
  
  import 'package:kernel/kernel.dart'
      show CanonicalName, Component, NonNullableByDefaultCompiledMode;
  
  import 'base/nnbd_mode.dart';
  
  import 'base/processed_options.dart' show ProcessedOptions;
  
  