anchor.diff:  
     if (primaryLibrary != null) {
       var primaryPlatform = libraries[primaryLibrary];
  -    if (primaryPlatform.worksEverywhere) {
  -      return new DartPlatform.universal(
  -          reason: 'primary library - `$primaryLibrary`');
  -    }
  -    if (primaryPlatform.restrictedTo != null &&
  -        primaryPlatform.restrictedTo.length == 1) {
  +    if (primaryPlatform.restrictedTo?.length == 1) {
         return new DartPlatform.withRestrictions(primaryPlatform.restrictedTo,
             reason: 'primary library - `$primaryLibrary`');
       }
  
positive.code:   if (transitiveLibs == null) {
      return new DartPlatform.conflict('failed to scan transitive libraries');
    }
    final libraries = new Map.fromIterable(transitiveLibs.keys ?? <String>[],
        value: (key) => classifyLibPlatform(transitiveLibs[key]));
    final primaryLibrary =
        _selectPrimaryLibrary(pubspec, transitiveLibs.keys.toSet());
  
negative.code:     return new DartPlatform.universal(reason: 'All libraries agree');
    }
  
    if (primaryLibrary != null && libraries[primaryLibrary].worksEverywhere) {
      return new DartPlatform.universal(
          reason: 'primary library - `$primaryLibrary`');
    }
  
    final items = libraries.values
        .where((p) => !p.worksEverywhere)
        .expand((p) => p.restrictedTo)
  