- An explicit API for initializing the implicit view (aka `window`).
- The explicit API is being called during the engine initialization for now, but we could simply remove that or make it conditional.
- Remove direct usages of `window` in tests:
- Most of the usages were being delegated to `PlatformDispatcher` anyway (e.g. `sendPlatformMessage`).
- This makes it **_clearer_** which tests depend on the implicit view (there are still hidden/indirect dependencies though).
Part of https://github.com/flutter/flutter/issues/134443