"Flutter is optimized for 2D mobile apps that want to run in both Android and iOS. Apps that use Material Design are particularly well suited for Flutter."
Which means that, if Flutter succeeds, iOS users should expect to see more and more apps built using Google's Material Design language. It's already happening for some of Google's iOS apps--the floating action button in Calendar, Hangouts, Photos, Docs, the iconset in most of their iOS apps (Gmail and Chrome are partial exceptions), etc.
In other words, Material Design is making significant inroads into iOS. But Apple's design language isn't doing the same in Android world.
Disclaimer: I'm a cofounder of Recent News (https://recent.io/), and we borrowed some Material Design concepts for the iOS version of our app.