Catch async functions passed to setState(). (#3992)

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

@@ -416,7 +416,22 @@ abstract class State<T extends StatefulWidget> {
       }
       return true;
     });
-    fn();
+    dynamic result = fn() as dynamic;
+    assert(() {
+      if (result is Future) {
+        throw new FlutterError(
+          'setState() callback argument returned a Future.\n'
+          'The setState() method on $this was invoked with a closure or method that '
+          'returned a Future. Maybe it is marked as "async".\n'
+          'Instead of performing asynchronous work inside a call to setState(), first '
+          'execute the work (without updating the widget state), and then synchronously '
+          'update the state inside a call to setState().'
+        );
+      }
+      // We ignore other types of return values so that you can do things like:
+      //   setState(() => x = 3);
+      return true;
+    });
     _element.markNeedsBuild();
   }
 

packages/flutter/test/widget/set_state_4_test.dart

@@ -0,0 +1,30 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+class Changer extends StatefulWidget {
+  @override
+  ChangerState createState() => new ChangerState();
+}
+
+class ChangerState extends State<Changer> {
+  void test0() { setState(() { }); }
+  void test1() { setState(() => 1); }
+  void test2() { setState(() async { }); }
+
+  @override
+  Widget build(BuildContext context) => new Text('test');
+}
+
+void main() {
+  testWidgets('setState() catches being used with an async callback', (WidgetTester tester) async {
+    await tester.pumpWidget(new Changer());
+    ChangerState s = tester.state(find.byType(Changer));
+    expect(s.test0, isNot(throws));
+    expect(s.test1, isNot(throws));
+    expect(s.test2, throws);
+  });
+}