More emphasis on `dart:js` to JS Interop page notice

[MaryaBelanger]

sdk/sdk/lib/js/js.dart

Line 7 in 2c0a5f4

/// You should usually use `package:js` instead of this library. For more

Isn't dart:js is essentially deprecated / soft deprecated? If people really shouldn't be using it at all, the wording "you should usually..." needs to be a little more direct. (sparked from this related comment)

If there are specific use cases that only dart:js can address, that will not be addressed by the JS Interop updates coming up in the next couple releases, then those should be clearly listed at the top. I'm imagining something like:

dart:js interop functionality has mostly been replaced by the package:js library. Use package:js for JavaScript interop, unless you want to .... (list of things only dart:js can do)