[wildcards] UNUSED_FIELD tests

Since `_` fields are binding, I think we want to treat `_`s like any other private field and report `UNUSED_FIELD`s accordingly.

These tests confirm that our current implementation is WAI.

Fixes: #55862

Change-Id: I2c8486a3b9e55d22c53989d74de974334477ec33
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/373405
Commit-Queue: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/analyzer/test/src/diagnostics/unused_field_test.dart

@@ -288,6 +288,29 @@ class A {
     ]);
   }
 
+  test_notUsed_noReference_wildcard() async {
+    await assertErrorsInCode(r'''
+class A {
+  int _ = 0;
+}
+''', [
+      error(WarningCode.UNUSED_FIELD, 16, 1),
+    ]);
+  }
+
+  test_notUsed_noReference_wildcard_preWildcards() async {
+    await assertErrorsInCode(r'''
+// @dart = 3.4
+// (pre wildcard-variables)
+
+class A {
+  int _ = 0;
+}
+''', [
+      error(WarningCode.UNUSED_FIELD, 60, 1),
+    ]);
+  }
+
   test_notUsed_nullAssign() async {
     await assertNoErrorsInCode(r'''
 class A {