Throw FlutterError when calling setState in State constructor (#16759)

* log error when calling setState in constructor

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

@@ -1103,6 +1103,15 @@ abstract class State<T extends StatefulWidget> extends Diagnosticable {
           'consider breaking the reference to this object during dispose().'
         );
       }
+      if (_debugLifecycleState == _StateLifecycle.created && !mounted) {
+        throw new FlutterError(
+          'setState() called in constructor: $this\n'
+          'This happens when you call setState() on a State object for a widget that '
+          'hasn\'t been inserted into the widget tree yet. It is not necessary to call '
+          'setState() in the constructor, since the state is already assumed to be dirty '
+          'when it is initially created.'
+        );
+      }
       return true;
     }());
     final dynamic result = fn() as dynamic;

packages/flutter/test/widgets/set_state_5_test.dart

@@ -0,0 +1,34 @@
+// Copyright 2018 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 BadWidget extends StatefulWidget {
+  @override
+  State<StatefulWidget> createState() => new BadWidgetState();
+
+}
+
+class BadWidgetState extends State<BadWidget> {
+  BadWidgetState() {
+    setState(() {
+     _count = 1;
+    });
+  }
+  
+  int _count = 0;
+
+  @override
+  Widget build(BuildContext context) {
+    return new Text(_count.toString());
+  }
+}
+
+void main() {
+  testWidgets('setState() catches being used inside a constructor', (WidgetTester tester) async {
+    await tester.pumpWidget(new BadWidget());
+    expect(tester.takeException(), const isInstanceOf<FlutterError>());
+  });
+}