avoid returning null for Future (#23784)

dev/devicelab/lib/framework/utils.dart

@@ -169,7 +169,7 @@ Future<String> getDartVersion() async {
 
 Future<String> getCurrentFlutterRepoCommit() {
   if (!dir('${flutterDirectory.path}/.git').existsSync()) {
-    return null;
+    return Future<String>.value(null);
   }
 
   return inDirectory<String>(flutterDirectory, () {

examples/flutter_gallery/test/example_code_parser_test.dart

@@ -41,12 +41,12 @@ test 1 1
 
 class TestAssetBundle extends AssetBundle {
   @override
-  Future<ByteData> load(String key) => null;
+  Future<ByteData> load(String key) async => null;
 
   @override
-  Future<String> loadString(String key, { bool cache = true }) {
+  Future<String> loadString(String key, { bool cache = true }) async {
     if (key == 'lib/gallery/example_code.dart')
-      return Future<String>.value(testCodeFile);
+      return testCodeFile;
     return null;
   }
 

packages/flutter/lib/src/painting/image_resolution.dart

@@ -217,7 +217,7 @@ class AssetImage extends AssetBundleImageProvider {
 
   static Future<Map<String, List<String>>> _manifestParser(String jsonData) {
     if (jsonData == null)
-      return null;
+      return SynchronousFuture<Map<String, List<String>>>(null);
     // TODO(ianh): JSON decoding really shouldn't be on the main thread.
     final Map<String, dynamic> parsedJson = json.decode(jsonData);
     final Iterable<String> keys = parsedJson.keys;

packages/flutter/lib/src/scheduler/binding.dart

@@ -250,7 +250,7 @@ mixin SchedulerBinding on BindingBase, ServicesBinding {
     }
   }
 
-  Future<String> _handleLifecycleMessage(String message) {
+  Future<String> _handleLifecycleMessage(String message) async {
     handleAppLifecycleStateChanged(_parseAppLifecycleMessage(message));
     return null;
   }

packages/flutter/test/widgets/image_resolution_test.dart

@@ -86,7 +86,7 @@ class TestAssetBundle extends CachingAssetBundle {
   Future<String> loadString(String key, { bool cache = true }) {
     if (key == 'AssetManifest.json')
       return SynchronousFuture<String>(manifest);
-    return null;
+    return SynchronousFuture<String>(null);
   }
 
   @override

packages/flutter_test/lib/src/binding.dart

@@ -1600,7 +1600,7 @@ class _MockHttpRequest extends HttpClientRequest {
   List<Cookie> get cookies => null;
 
   @override
-  Future<HttpClientResponse> get done => null;
+  Future<HttpClientResponse> get done async => null;
 
   @override
   Future<void> flush() {

packages/flutter_tools/lib/src/commands/config.dart

@@ -59,7 +59,7 @@ class ConfigCommand extends FlutterCommand {
 
   /// Return null to disable analytics recording of the `config` command.
   @override
-  Future<String> get usagePath => null;
+  Future<String> get usagePath async => null;
 
   @override
   Future<FlutterCommandResult> runCommand() async {

packages/flutter_tools/lib/src/commands/shell_completion.dart

@@ -39,7 +39,7 @@ class ShellCompletionCommand extends FlutterCommand {
 
   /// Return null to disable analytics recording of the `bash-completion` command.
   @override
-  Future<String> get usagePath => null;
+  Future<String> get usagePath async => null;
 
   @override
   Future<FlutterCommandResult> runCommand() async {

packages/flutter_tools/lib/src/tester/flutter_tester.dart

@@ -245,5 +245,5 @@ class _NoopPortForwarder extends DevicePortForwarder {
   List<ForwardedPort> get forwardedPorts => <ForwardedPort>[];
 
   @override
-  Future<void> unforward(ForwardedPort forwardedPort) => null;
+  Future<void> unforward(ForwardedPort forwardedPort) async { }
 }

packages/flutter_tools/lib/src/vmservice.dart

@@ -951,7 +951,7 @@ class VM extends ServiceObjectOwner {
 
   Future<void> refreshViews() {
     if (!isFlutterEngine)
-      return null;
+      return Future<void>.value();
     _viewCache.clear();
     // Send one per-application request that refreshes all views in the app.
     return vmService.vm.invokeRpc<ServiceObject>('_flutter.listViews', timeout: kLongRequestTimeout);

packages/flutter_tools/test/dart/pub_get_test.dart

@@ -213,7 +213,7 @@ class MockStreamSubscription<T> implements StreamSubscription<T> {
   Future<E> asFuture<E>([E futureValue]) => Future<E>.value();
 
   @override
-  Future<void> cancel() => null;
+  Future<void> cancel() async { }
 
   @override
   dynamic noSuchMethod(Invocation invocation) => null;

packages/flutter_tools/test/resident_runner_test.dart

@@ -18,10 +18,10 @@ class TestRunner extends ResidentRunner {
   String receivedCommand;
 
   @override
-  Future<void> cleanupAfterSignal() => null;
+  Future<void> cleanupAfterSignal() async { }
 
   @override
-  Future<void> cleanupAtFinish() => null;
+  Future<void> cleanupAtFinish() async { }
 
   @override
   Future<void> handleTerminalCommand(String code) async {
@@ -39,7 +39,7 @@ class TestRunner extends ResidentRunner {
     Completer<void> appStartedCompleter,
     String route,
     bool shouldBuild = true,
-  }) => null;
+  }) async => null;
 }
 
 void main() {