anchor.diff:  
   /// Mixin containing helper methods for testing summary resynthesis.  Intended
   /// to be applied to a class implementing [ResynthesizeTestStrategy].
  -mixin ResynthesizeTestHelpers implements ResynthesizeTestStrategy {
  +mixin ResynthesizeTestHelpers
  +    implements ResynthesizeTestStrategy, ResourceProviderMixin {
     Future<LibraryElementImpl /*!*/ > checkLibrary(String text,
         {bool allowErrors = false, bool dumpSummaries = false}) async {
       throw 42;
  
positive.code:     var library = await checkLibrary('@a import "foo.dart"; const a = b;');
      checkElementText(
          library,
          r'''
  import 'foo.dart';
    metadata
      Annotation
  
negative.code: const int a;
    constantInitializer
      SimpleIdentifier
        staticElement: ${toUriStr('/foo.dart')}::@getter::b
        staticType: int
        token: b
  ''',
  