Adds comment

src/harness/unittests/languageService.ts

@@ -17,6 +17,8 @@ class Carousel<T> extends Vue {
             "vue-class-component.d.ts": `import Vue from "./vue";
 export function Component(x: Config): any;`
 };
+        // Regression test for GH #18245 - bug in single line comment writer caused a debug assertion when attempting
+        //  to write an alias to a module's default export was referrenced across files and had no default export
         it("should be able to create a language service which can respond to deinition requests without throwing", () => {
             const languageService = ts.createLanguageService({
                 getCompilationSettings() {