anchor.diff:      if (_symbolConstructorTarget == null) {
         if (_symbolImplementationClass == null) {
           _symbolImplementationClass =
  -            _findClass(internalLibrary, 'Symbol', required: false);
  +            _findClassOrNull(internalLibrary, 'Symbol');
         }
         if (_symbolImplementationClass != null) {
  -        _symbolConstructorTarget =
  -            _findConstructor(_symbolImplementationClass!, '', required: false);
  +        _symbolConstructorTarget = _env.lookupConstructor(
  +            _symbolImplementationClass!, '',
  +            required: false);
         }
       }
       if (_symbolClass == null) {
  -      _symbolClass = _findClass(coreLibrary, 'Symbol', required: false);
  +      _symbolClass = _findClassOrNull(coreLibrary, 'Symbol');
       }
       if (_symbolClass == null) {
         return;
       }
       _symbolConstructorImplementationTarget =
  -        _findConstructor(symbolClass, '', required: false);
  +        _env.lookupConstructor(symbolClass, '', required: false);
     }
   
     /// Whether [element] is the same as [symbolConstructor].
  
positive.code: 
    late final FunctionEntity checkTypeBound = _findRtiFunction('checkTypeBound');
  
    ClassEntity get _rtiImplClass => _findClass(rtiLibrary, 'Rti')!;
  
    ClassEntity get _rtiUniverseClass => _findClass(rtiLibrary, '_Universe')!;
  
    FieldEntity _findRtiClassField(String name) =>
        _findClassMember(_rtiImplClass, name)!;
  
    late final FieldEntity rtiAsField = _findRtiClassField('_as');
  
  
negative.code: 
    // From dart:async
    ClassEntity _findAsyncHelperClass(String name) =>
        _findClass(asyncLibrary, name);
  
    FunctionEntity _findAsyncHelperFunction(String name) =>
        _findLibraryMember(asyncLibrary, name)!;
  