Allow singleline string writer to be recursively used (#18297)

* Allow singleline string writer to be recursively used

* Add unit test exposing issue

* Fix lints

Jakefile.js

@@ -143,6 +143,7 @@ var harnessSources = harnessCoreSources.concat([
     "customTransforms.ts",
     "programMissingFiles.ts",
     "symbolWalker.ts",
+    "languageService.ts",
 ].map(function (f) {
     return path.join(unittestsDirectory, f);
 })).concat([