[dart2wasm] make dart.library.js_util false on conditional imports

[sigmundch]

Follow up from #55266

Eventually we may want to fully remove dart:js_util. It's already banned from most uses, but there are a few usages that need to be removed before it can be fully disallowed.

Meanwhile, I think we can be in the middleground where the direct import is allowed, but a conditional import is not.

[dart-github-bot]

Summary: The dart:js_util library is currently banned from most uses, but some remaining usages need to be removed before it can be fully disallowed. This issue proposes a middle ground where direct imports are allowed, but conditional imports are not.