Make PestoDemoState.scaffoldKey non-static so Favorites can instantiate a second PestoDemo (#5005)

Fixes https://github.com/flutter/flutter/issues/4996

examples/flutter_gallery/lib/demo/pesto_demo.dart

@@ -51,7 +51,7 @@ class PestoDemo extends StatefulWidget {
 }
 
 class _PestoDemoState extends State<PestoDemo> {
-  static final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
+  final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
   final TextStyle favoritesMessageStyle = _textStyle(16.0);
   final TextStyle userStyle = _textStyle(12.0, FontWeight.bold);
   final TextStyle emailStyle = _textStyle(12.0).copyWith(color: Colors.black54);