From ae62a3cf1b747a9cbb74957ac176e692ee045be5 Mon Sep 17 00:00:00 2001
From: Kate Lovett <katelovett@google.com>
Date: Fri, 15 Nov 2019 13:05:18 -0800
Subject: [PATCH] Error Message for createState assertion (#44610)

* Init

* Added error message to assert

* Removing unrelated changes
---
 packages/flutter/lib/src/widgets/framework.dart | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/packages/flutter/lib/src/widgets/framework.dart b/packages/flutter/lib/src/widgets/framework.dart
index d8336cfceb..f46e282bce 100644
--- a/packages/flutter/lib/src/widgets/framework.dart
+++ b/packages/flutter/lib/src/widgets/framework.dart
@@ -4055,7 +4055,12 @@ class StatefulElement extends ComponentElement {
     }());
     assert(_state._element == null);
     _state._element = this;
-    assert(_state._widget == null);
+    assert(
+      _state._widget == null,
+      'The createState function for $widget returned an old or invalid state '
+      'instance: ${_state._widget}, which is not null, violating the contract '
+      'for createState.',
+    );
     _state._widget = widget;
     assert(_state._debugLifecycleState == _StateLifecycle.created);
   }