In Nativescript, Code is not compiled to native code , it runs on separate thread that communites code with native component. The runtimes enable you to call APIs in the Android and iOS frameworks using JavaScript code. To do that they use JavaScript Virtual Machines - Google’s V8 for Android and WebKit’s JavaScriptCore implementation distributed with iOS 7.0+. While Flutter compiles code to ARM C/C++ library that is consumed by native components so may perform better.
I didn't get a chance to compare performance for the two, as for NativeScript you write code in Typescript/Javascript while Dart is used for Flutter and I'm very new to Dart.
NativeScript is an open source that Angular has also listed in resources and there is very good community for NativeScript now. If you are interested to list pro and cons of different frameworks, here is one very good article.
Note :Test NativeScript apps online with a code editor and run on your device. You can use either Angular or Pure JavaScript orTypeScript or Vue.js flavor to build the app.
On a Side note :Google has unveiled Flutter at the 2015 Dart developer summit, and NativeScript was backed by Progress from 2014. Maybe Google wanted to give developers others options to explore native applications with other frameworks.
If you look for available commands for Flutter, there is one command called fuchsia_reload to reload the operating new operating system Fuchsia that Google is working on, so it could be possible they want to provide early support for that.
UPDATE:
If we want to share the same codebase for web and phone, NativeScript is an obvious choice. The Angular and NativeScript teams teamed up to create nativescript-schematics, a schematic collection that enables you to build both web and mobile apps from a single project.
You can install it using
npm i -g @nativescript/schematics
And even you can migrate from existing project to code sharing projects as described here.