Use a member function instead of an anonymous closure

It's slightly more efficient.

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

@@ -1377,9 +1377,11 @@ class StatefulComponentElement<T extends StatefulComponent, U extends State<T>>
 
 abstract class _ProxyElement<T extends _ProxyComponent> extends ComponentElement<T> {
   _ProxyElement(T widget) : super(widget) {
-    _builder = (BuildContext context) => this.widget.child;
+    _builder = _build;
   }
 
+  Widget _build(BuildContext context) => widget.child;
+
   void update(T newWidget) {
     T oldWidget = widget;
     assert(widget != null);