enable lint avoid_void_async (#21652)

analysis_options.yaml

@@ -74,7 +74,7 @@ linter:
     - avoid_types_as_parameter_names
     # - avoid_types_on_closure_parameters # conflicts with always_specify_types
     - avoid_unused_constructor_parameters
-    # - avoid_void_async # not yet tested
+    - avoid_void_async
     - await_only_futures
     - camel_case_types
     - cancel_subscriptions

dev/devicelab/bin/tasks/android_semantics_integration_test.dart

@@ -6,7 +6,7 @@ import 'package:flutter_devicelab/framework/adb.dart';
 import 'package:flutter_devicelab/framework/framework.dart';
 import 'package:flutter_devicelab/tasks/integration_tests.dart';
 
-void main() async {
+Future<void> main() async {
   deviceOperatingSystem = DeviceOperatingSystem.android;
   await task(createAndroidSemanticsIntegrationTest());
 }

dev/devicelab/bin/tasks/gradle_plugin_test.dart

@@ -36,7 +36,7 @@ Future<void> runPluginProjectTest(Future<void> testFunction(FlutterPluginProject
   }
 }
 
-void main() async {
+Future<void> main() async {
   await task(() async {
     section('Find Java');
 

packages/flutter/test/widgets/async_lifecycle_test.dart

@@ -10,7 +10,7 @@ class InvalidOnInitLifecycleWidget extends StatefulWidget {
 
 class InvalidOnInitLifecycleWidgetState extends State<InvalidOnInitLifecycleWidget> {
   @override
-  void initState() async {
+  Future<void> initState() async {
     super.initState();
   }
 
@@ -31,7 +31,7 @@ class InvalidDidUpdateWidgetLifecycleWidget extends StatefulWidget {
 
 class InvalidDidUpdateWidgetLifecycleWidgetState extends State<InvalidDidUpdateWidgetLifecycleWidget> {
   @override
-  void didUpdateWidget(InvalidDidUpdateWidgetLifecycleWidget oldWidget) async {
+  Future<void> didUpdateWidget(InvalidDidUpdateWidgetLifecycleWidget oldWidget) async {
     super.didUpdateWidget(oldWidget);
   }
 

packages/flutter_test/test/test_config/flutter_test_config.dart

@@ -4,7 +4,7 @@
 
 import 'dart:async';
 
-void main(FutureOr<void> testMain()) async {
+Future<void> main(FutureOr<void> testMain()) async {
   await runZoned<dynamic>(testMain, zoneValues: <Type, String>{
     String: '/test_config',
   });

packages/flutter_test/test/test_config/nested_config/flutter_test_config.dart

@@ -4,7 +4,7 @@
 
 import 'dart:async';
 
-void main(FutureOr<void> testMain()) async {
+Future<void> main(FutureOr<void> testMain()) async {
   await runZoned<dynamic>(testMain, zoneValues: <Type, dynamic>{
     String: '/test_config/nested_config',
     int: 123,

packages/flutter_tools/lib/src/compile.dart

@@ -316,7 +316,7 @@ class ResidentCompiler {
 
   final List<_CompilationRequest> _compilationQueue = <_CompilationRequest>[];
 
-  void _handleCompilationRequest(_CompilationRequest request) async {
+  Future<void> _handleCompilationRequest(_CompilationRequest request) async {
     final bool isEmpty = _compilationQueue.isEmpty;
     _compilationQueue.add(request);
     // Only trigger processing if queue was empty - i.e. no other requests

packages/flutter_tools/test/flutter_manifest_test.dart

@@ -503,7 +503,7 @@ flutter:
   });
 
   group('FlutterManifest with MemoryFileSystem', () {
-    void assertSchemaIsReadable() async {
+    Future<void> assertSchemaIsReadable() async {
       const String manifest = '''
 name: test
 dependencies:
@@ -529,18 +529,18 @@ flutter:
       );
     }
 
-    testUsingContext('Validate manifest on original fs', () async {
+    testUsingContext('Validate manifest on original fs', () {
       assertSchemaIsReadable();
     });
 
     testUsingContextAndFs('Validate manifest on Posix FS',
-        new MemoryFileSystem(style: FileSystemStyle.posix), () async {
+        new MemoryFileSystem(style: FileSystemStyle.posix), () {
           assertSchemaIsReadable();
         }
     );
 
     testUsingContextAndFs('Validate manifest on Windows FS',
-        new MemoryFileSystem(style: FileSystemStyle.windows), () async {
+        new MemoryFileSystem(style: FileSystemStyle.windows), () {
           assertSchemaIsReadable();
         }
     );