[lint] more tests for unnecessary_underscores

Change-Id: I13919d11d3f782d3c80682025113d0cea9f27a30
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/403440
Commit-Queue: Phil Quitslund <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: pq

pkg/linter/test/rules/unnecessary_underscores_test.dart

@@ -17,6 +17,17 @@ class UnnecessaryUnderscoresTest extends LintRuleTest {
   @override
   String get lintRule => 'unnecessary_underscores';
 
+  test_enum_field_unused() async {
+    await assertDiagnostics(r'''
+enum E {
+  __,
+}
+''', [
+      // No lint.
+      error(WarningCode.UNUSED_FIELD, 11, 2),
+    ]);
+  }
+
   test_field_unused() async {
     await assertDiagnostics(r'''
 class C {
@@ -89,6 +100,30 @@ void f() {
 ''');
   }
 
+  test_localFunction_parameter_unused() async {
+    await assertDiagnostics(r'''
+void f() {
+  g(int __) {}
+}
+''', [
+      error(WarningCode.UNUSED_ELEMENT, 13, 1),
+      lint(19, 2),
+    ]);
+  }
+
+  test_localFunction_parameter_used() async {
+    await assertDiagnostics(r'''
+void f() {
+  g(int __) {
+    print(__);
+  }
+}
+''', [
+      // No lint.
+      error(WarningCode.UNUSED_ELEMENT, 13, 1),
+    ]);
+  }
+
   test_localFunction_unused() async {
     await assertDiagnostics(r'''
 f() {