Check that State.initState and State.didUpdateWidget don't return Futures (#15183)

* add a debug check that the returned Type of State.initState and State.didUpdateWidget are not Futures

* add test to verify that async lifecycles throw FlutterErrors

* address some feedback, match implementation of setState check

* address feedback from hixie

* fix odd spacing in test

packages/flutter/lib/src/widgets/framework.dart

@@ -3731,7 +3731,18 @@ class StatefulElement extends ComponentElement {
     assert(_state._debugLifecycleState == _StateLifecycle.created);
     try {
       _debugSetAllowIgnoredCallsToMarkNeedsBuild(true);
-      _state.initState();
+      final dynamic debugCheckForReturnedFuture = _state.initState() as dynamic;
+      assert(() {
+        if (debugCheckForReturnedFuture is Future) {
+          throw new FlutterError(
+            '${_state.runtimeType}.initState() returned a Future.\n'
+            'State.initState() must be a void method without an `async` keyword.\n'
+            'Rather than awaiting on asynchronous work directly inside of initState,\n'
+            'call a separate method to do this work without awaiting it.'
+          );
+        }
+        return true;
+      }());
     } finally {
       _debugSetAllowIgnoredCallsToMarkNeedsBuild(false);
     }
@@ -3753,7 +3764,18 @@ class StatefulElement extends ComponentElement {
     _state._widget = widget;
     try {
       _debugSetAllowIgnoredCallsToMarkNeedsBuild(true);
-      _state.didUpdateWidget(oldWidget);
+      final dynamic debugCheckForReturnedFuture = _state.didUpdateWidget(oldWidget) as dynamic;
+      assert(() {
+        if (debugCheckForReturnedFuture is Future) {
+          throw new FlutterError(
+            '${_state.runtimeType}.didUpdateWidget() returned a Future.\n'
+            'State.didUpdateWidget() must be a void method without an `async` keyword.\n'
+            'Rather than awaiting on asynchronous work directly inside of didUpdateWidget,\n'
+            'call a separate method to do this work without awaiting it.'
+          );
+        }
+        return true;
+      }());
     } finally {
       _debugSetAllowIgnoredCallsToMarkNeedsBuild(false);
     }

packages/flutter/test/widgets/async_lifecycle_test.dart

+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+class InvalidOnInitLifecycleWidget extends StatefulWidget {
+  const InvalidOnInitLifecycleWidget({Key key}) : super(key: key);
+
+  @override
+  InvalidOnInitLifecycleWidgetState createState() => new InvalidOnInitLifecycleWidgetState();
+}
+
+class InvalidOnInitLifecycleWidgetState extends State<InvalidOnInitLifecycleWidget> {
+  @override
+  void initState() async {
+    super.initState();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new Container();
+  }
+}
+
+class InvalidDidUpdateWidgetLifecycleWidget extends StatefulWidget {
+  const InvalidDidUpdateWidgetLifecycleWidget({Key key, this.id}) : super(key: key);
+
+  final int id;
+
+  @override
+  InvalidDidUpdateWidgetLifecycleWidgetState createState() => new InvalidDidUpdateWidgetLifecycleWidgetState();
+}
+
+class InvalidDidUpdateWidgetLifecycleWidgetState extends State<InvalidDidUpdateWidgetLifecycleWidget> {
+  @override
+  void didUpdateWidget(InvalidDidUpdateWidgetLifecycleWidget oldWidget) async {
+    super.didUpdateWidget(oldWidget);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new Container();
+  }
+}
+
+void main() {
+  testWidgets('async onInit throws FlutterError', (WidgetTester tester) async {
+    await tester.pumpWidget(const InvalidOnInitLifecycleWidget());
+
+    expect(tester.takeException(), isFlutterError);
+  });
+
+  testWidgets('async didUpdateWidget throws FlutterError', (WidgetTester tester) async {
+    await tester.pumpWidget(const InvalidDidUpdateWidgetLifecycleWidget(id: 1));
+    await tester.pumpWidget(const InvalidDidUpdateWidgetLifecycleWidget(id: 2));
+
+    expect(tester.takeException(), isFlutterError);
+  });
+}