anchor.diff:    @override
     ir.Member get node;
   
  -  StaticTypeCache staticTypes;
  +  StaticTypeCache? staticTypes;
   
     Iterable<ConstantValue> getMetadata(IrToElementMap elementMap);
   
  -  InterfaceType getMemberThisType(JsToElementMap elementMap);
  +  InterfaceType? getMemberThisType(JsToElementMap elementMap);
   
     ClassTypeVariableAccess get classTypeVariableAccess;
   
  
positive.code: 
    @override
    void forEachParameter(JsToElementMap elementMap,
        void f(DartType type, String name, ConstantValue defaultValue)) {
      void handleParameter(ir.VariableDeclaration parameter,
          {bool isOptional = true}) {
        DartType type = elementMap.getDartType(parameter.type);
        String name = parameter.name;
        ConstantValue defaultValue;
        if (isOptional) {
          if (parameter.initializer != null) {
            defaultValue =
  
negative.code: 
  /// Environment for fast lookup of component libraries.
  class KProgramEnv implements interfaces.KProgramEnv {
    final Set<ir.Component> _components = {};
  
    late final Map<Uri, KLibraryEnv> _libraryMap = {
      for (final component in _components)
        for (final library in component.libraries)
          library.importUri: KLibraryEnv(library),
    };
  
    /// TODO(johnniwinther): Handle arbitrary load order if needed.
    ir.Member? get mainMethod => mainComponent.mainMethod;
  
    ir.Component get mainComponent => _components.first;
  
    void addComponent(ir.Component component) {
      if (_components.add(component)) {
        for (ir.Library library in component.libraries) {
          _libraryMap[library.importUri] ??= KLibraryEnv(library);
        }
      }
    }
  
    /// Return the [KLibraryEnv] for the library with the canonical [uri].
    KLibraryEnv? lookupLibrary(Uri uri) => _libraryMap[uri];
  
    /// Calls [f] for each library in this environment.
    void forEachLibrary(void f(KLibraryEnv library)) {
      _libraryMap.values.forEach(f);
    }
  
    /// Returns the number of libraries in this environment.
    int get length => _libraryMap.length;
  
    /// Convert this [KProgramEnv] to the corresponding [JProgramEnv].
    @override
  