dart.library.js_util -> dart.library.js_interop for web detection

dart2wasm will no longer support `dart:js_util`, so
`dart.library.js_util` will be insufficient for detecting web backends.
`dart:js_interop` is intended to replace it for the foreseeable future,
so until we have an environment variable that more directly reflects
whether the code is compiled with a web backend, we can use
`dart.library.js_interop`.

Bug: https://github.com/dart-lang/sdk/issues/61550
Change-Id: I6a6a69642e6c471e5cab3b1417255a4748b8d072
Reviewed-on: https://dart-review.googlesource.com/c/flute/+/451680
Commit-Queue: Ömer Ağacan <omersa@google.com>
Auto-Submit: Mayank Patke <fishythefish@google.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>