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`.

Other mentions of `dart.library.js_util` in tests are benign and aren't
designed to detect all web backends specifically.

Bug: #61550
Change-Id: I6a6a69644c3614b3693ad72b546e111217575bfb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/451547
Commit-Queue: Mayank Patke <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: fishythefish

pkg/analyzer/test/src/lint/linter/linter_context_impl_test.dart

@@ -285,7 +285,7 @@ f<U>() => A<U>();
 
   void test_true_argument_list_nonBool() async {
     await resolve('''
-const bool kIsWeb = bool.fromEnvironment('dart.library.js_util');
+const bool kIsWeb = bool.fromEnvironment('dart.library.js_interop');
 class A {
   const A(List<int> l);
 }

tests/language/library/env_test.dart

@@ -37,10 +37,10 @@ main() {
     const bool.fromEnvironment("dart.library.web_gl"),
   );
 
-  // All web backends support `dart:js_util`
+  // All web backends support `dart:js_interop`
   Expect.equals(
     isWebConfiguration,
-    const bool.fromEnvironment("dart.library.js_util"),
+    const bool.fromEnvironment("dart.library.js_interop"),
   );
 
   // Web platforms override 'dart.library.io' to return "false".