anchor.diff:  
     /// Holds JS type generators keyed by their underlying DartType.
     final typeContainer = ModuleItemContainer<DartType>.asObject('T',
  -      keyToString: (DartType t) => escapeIdentifier(_typeString(t)));
  +      keyToString: (DartType t) => escapeIdentifier(_typeString(t))!);
   
     final js_ast.Expression Function(String, [List<Object>]) _runtimeCall;
   
  
positive.code:       // TODO(40273) Remove prepended text when we have a better way to hide
        // these names from debug tools.
        _unboundTypeIds[type] =
            js_ast.TemporaryId(escapeIdentifier('__t\$${_typeString(type)}'));
      }
  
      for (var free in freeVariables) {
  
negative.code: 
    js_ast.Statement _dischargeFreeType(DartType type) {
      typeContainer.setNoEmit(type);
      var init = typeContainer[type]!;
      var id = _unboundTypeIds[type]!;
      // TODO(vsm): Change back to `let`.
      // See https://github.com/dart-lang/sdk/issues/40380.
      return js.statement('var # = () => ((# = #)());', [
  