anchor.diff:        return domains.putIfAbsent(reflector, () {
           _Capabilities capabilities =
               _capabilitiesOf(capabilityLibrary, reflector);
  +        assert(_isImportableLibrary(reflector.library, dataId, _resolver));
           importCollector._addLibrary(reflector.library);
           return new _ReflectorDomain(_resolver, dataId, reflector, capabilities);
         });
  
positive.code:     void addLibrary(LibraryElement library, ClassElement reflector) {
        _ReflectorDomain domain = getReflectorDomain(reflector);
        if (domain._capabilities.supportsLibraries) {
          importCollector._addLibrary(library);
          domain._libraries.add(library);
        }
  
negative.code:     for (LibraryElement library in _resolver.libraries) {
        for (ClassElement reflector
            in getReflectors(library.name, library.metadata)) {
          assert(_isImportableLibrary(reflector.library, dataId, _resolver));
          addLibrary(library, reflector);
        }
  
  