Migrate a Flutter web app from dart:html to package:web and dart:js_interop
A step-by-step migration off the deprecated dart:html, dart:js_util, and package:js onto package:web 1.1.1 and dart:js_interop: how to find every offending import with the dart2wasm compiler, what dart fix does and does not rename, the JSImmutableListWrapper and innerHTML traps, and how to verify with flutter build web --wasm.