mechanism to indicate package only supports dart and not flutter

Since flutter web stuff can reasonably import `dart:html` and use things like [`localStorage`](https://api.dart.dev/stable/2.19.6/dart-html/Window/localStorage.html) (I assume), we can't detect that a package isn't really compatible with Flutter Web, just because it imports `dart:html`.

We could try to detect if it uses the DOM parts of `dart:html`, but that's probably not a good idea, you can use those parts in a flutter web app to compute styles, or maybe do something else.

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We probably need a manual mechanism for indicating which SDKs a package is intended to be used with.