This PR adds:
* JS-interop types to expose addView/removeView from the running FlutterApp object on JavaScript.
* Also, the options object that can be passed to `addView`.
### Issues:
* Fixes: https://github.com/flutter/flutter/issues/137377
### Info
Most interesting files:
* `app_bootstrap.dart` -> Adds the implementation for JS add/remove view.
* `js_app.dart` -> Adds the js-interop layer for the FlutterApp object, and the configuration type. (Two options: `hostElement` and `initialData`).
* `flutter_view_manager.dart` -> An abstraction over the `viewData` map that keeps related things together: viewData, js configuration options, register/unregister methods and a Stream<void> of modification events.
The rest of the changes were ""required"" to have a small demo that does anything (currently it lets me "register" views from javascript). I didn't add much in there, because probably it's already being worked on by @mdebbar; just fiddled with the constructor of the EngineFlutterView to create views from JS Config, and added a wrapper around the `viewData` map (`FlutterViewManager`) to prevent direct access to the Map.
## Usage
This is how I'm currently initializing my Flutter App, so I can "leak" the `flutterApp` to window and do things asynchronously after flutter loads:
```html
<script>
window.addEventListener('load', async function(ev) {
_flutter.loader.loadEntrypoint({
onEntrypointLoaded: function(engineInitializer) {
engineInitializer.initializeEngine({
multiViewEnabled: true,
}).then(function(appRunner) {
return appRunner.runApp();
}).then(onAppRunning);
}
});
// Leak flutterApp to window so we can do async things...
function onAppRunning(flutterApp) {
console.log('Running app', flutterApp);
window.flutterApp = flutterApp;
}
});
</script>
```
This to test:
* Go on your JS console and use `flutterApp.addView({})` and see how that returns a Promise for the viewId it just added,
* Now do `flutterApp.removeView(0)` (removes the implicitView), and see how everything starts crashing! (as expected)
* Get out of this weird state with a hot reload ð¥³
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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