anchor.diff:          newModelElement = Field.inherited(
               e, enclosingContainer, library, packageGraph, getter, setter);
         }
  -    }
  -    if (e is TopLevelVariableElement) {
  +    } else if (e is TopLevelVariableElement) {
         assert(getter != null || setter != null);
         newModelElement =
             TopLevelVariable(e, library, packageGraph, getter, setter);
  +    } else {
  +      throw UnimplementedError(
  +          'Unrecognized property inducing element: $e (${e.runtimeType})');
       }
   
       if (enclosingContainer != null) assert(newModelElement is Inheritable);
       _cacheNewModelElement(e, newModelElement, library,
           enclosingContainer: enclosingContainer);
   
  -    assert(newModelElement!.element is! MultiplyInheritedExecutableElement);
  -    return newModelElement!;
  +    assert(newModelElement.element is! MultiplyInheritedExecutableElement);
  +    return newModelElement;
     }
   
     /// Creates a [ModelElement] from a non-property-inducing [e].
  
positive.code:   @override
    bool get hasDocumentationComment => element!.documentationComment != null;
  
    late final String _sourceCode =
        _sourceCodeRenderer.renderSourceCode(super.sourceCode);
  
    @override
    String get sourceCode => _sourceCode;
  
    @override
    int compareTo(dynamic other) {
  
negative.code:   /// that need to be documented.  See [Feature] for a list.
    Set<Feature> get features {
      return {
        ...annotations.whereNot((a) => _specialFeatures.contains(a.name)),
        // 'const' and 'static' are not needed here because 'const' and 'static'
        // elements get their own sections in the doc.
        if (isFinal) Feature.finalFeature,
  