Analyzer: unregister test rules in tearDown

Change-Id: I8aae2ce4d75af0e47b900a149ff73dcb6064a26e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/337613
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/analysis_server/test/src/services/completion/yaml/analysis_options_generator_test.dart

@@ -19,7 +19,7 @@ void main() {
 
 @reflectiveTest
 class AnalysisOptionsGeneratorTest extends YamlGeneratorTest {
-  // Keep track of any added rules so they can be unregistered at tearDown
+  // Keep track of any added rules so they can be unregistered in `tearDown`.
   var addedRules = <LintRule>[];
 
   @override

pkg/analysis_server/test/src/services/correction/fix/analysis_options/remove_lint_test.dart

@@ -28,9 +28,16 @@ class DeprecatedRule extends LintRule {
 
 @reflectiveTest
 class RemoveLintTest extends AnalysisOptionsFixTest {
+  // Keep track of this rule so it can be unregistered in `tearDown`.
+  var deprecatedRule = DeprecatedRule();
+
   void setUp() {
     registerLintRules();
-    Registry.ruleRegistry.register(DeprecatedRule());
+    Registry.ruleRegistry.register(deprecatedRule);
+  }
+
+  void tearDown() {
+    Registry.ruleRegistry.unregister(deprecatedRule);
   }
 
   Future<void> test_deprecated() async {