Previously, when the code contained `const HtmlElementView()` it would break even if it's guarded by `if (kIsWeb)`.

This PR makes it such that `const HtmlElementView()` is allowed but it still throws if it gets inserted into the widget tree by mistake on non-web platforms.

One improvement we can make in the future is to have a conditional import:
- `_html_element_view_web.dart` that contains the real `HtmlElementView` that can only be instantiated on web.
- `_html_element_view_io.dart` that contains a stub with an unimplemented `build()` method.

Fixes https://github.com/flutter/flutter/issues/43532