linter: enforce unnecessary_async

Change-Id: If5d025ccf2d1a6d80256579bee7a7b3a4d039dde
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/429061
Reviewed-by: Keerti Parthasarathy <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: srawlins

pkg/linter/analysis_options.yaml

@@ -48,6 +48,7 @@ linter:
     - test_types_in_equals
     - throw_in_finally
     - unawaited_futures # pedantic
+    - unnecessary_async
     - unnecessary_breaks
     - unnecessary_final
     - unnecessary_ignore

pkg/linter/test/rule_test_support.dart

@@ -264,11 +264,11 @@ class PubPackageResolutionTest with MockPackagesMixin, ResourceProviderMixin {
   }
 
   /// Asserts that there are no diagnostics in the given [content].
-  Future<void> assertNoDiagnostics(String content) async =>
+  Future<void> assertNoDiagnostics(String content) =>
       assertDiagnostics(content, const []);
 
   /// Asserts that there are no diagnostics in the file at the given [path].
-  Future<void> assertNoDiagnosticsInFile(String path) async =>
+  Future<void> assertNoDiagnosticsInFile(String path) =>
       assertDiagnosticsInFile(path, const []);
 
   /// Asserts that no diagnostics are reported when resolving [content].

pkg/linter/test/validate_incompatible_rules_test.dart

@@ -11,7 +11,7 @@ void main() {
     registerLintRules();
     for (var rule in Registry.ruleRegistry) {
       for (var incompatibleRule in rule.incompatibleRules) {
-        test(rule.name, () async {
+        test(rule.name, () {
           var referencedRule = Registry.ruleRegistry.firstWhere(
             (r) => r.name == incompatibleRule,
           );

pkg/linter/test/validate_no_rule_description_references_test.dart

@@ -16,7 +16,7 @@ void main() {
     () {
       registerLintRules();
       for (var rule in Registry.ruleRegistry) {
-        test(rule.name, () async {
+        test(rule.name, () {
           var result = Process.runSync('grep', [
             '-R',
             rule.description,

pkg/linter/test/verify_reflective_test_suites_test.dart

@@ -79,7 +79,7 @@ class _VerifyTests {
       }
     }
     var relativePath = path.relative(directory.path, from: testDirPath);
-    test(relativePath, () async {
+    test(relativePath, () {
       if (testFileNames.isEmpty) return;
       if (testAllFile == null) return;