This wraps up the platform view improvements discussed in https://github.com/flutter/flutter/issues/127030.

- Splits `HtmlElementView` into 2 files that are conditionally imported.
- The non-web version can be instantiated but it throws if it ends up being built in a widget tree.
- Out-of-the-box view factories that create visible & invisible DOM elements given a `tagName` parameter.
- New `HtmlElementView.fromTagName()` constructor that uses the default factories to create DOM elements.
- Tests covering the new API.

Depends on https://github.com/flutter/engine/pull/43828

Fixes https://github.com/flutter/flutter/issues/127030