enable lint avoid_void_async (#21652)

packages/flutter_tools/lib/src/compile.dart

diff --git a/packages/flutter_tools/lib/src/compile.dart b/packages/flutter_tools/lib/src/compile.dart
index fb8e251..7f37282 100644
--- a/packages/flutter_tools/lib/src/compile.dart
+++ b/packages/flutter_tools/lib/src/compile.dart
@@ -316,7 +316,7 @@
 
   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

diff --git a/packages/flutter_tools/test/flutter_manifest_test.dart b/packages/flutter_tools/test/flutter_manifest_test.dart
index 0733412..2c29651 100644
--- a/packages/flutter_tools/test/flutter_manifest_test.dart
+++ b/packages/flutter_tools/test/flutter_manifest_test.dart
@@ -503,7 +503,7 @@
   });
 
   group('FlutterManifest with MemoryFileSystem', () {
-    void assertSchemaIsReadable() async {
+    Future<void> assertSchemaIsReadable() async {
       const String manifest = '''
 name: test
 dependencies:
@@ -529,18 +529,18 @@
       );
     }
 
-    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();
         }
     );