[rename fixit] *Component* -> *Widget*

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308
diff --git a/examples/layers/widgets/spinning_square.dart b/examples/layers/widgets/spinning_square.dart
index cd0adc1..bf754c6 100644
--- a/examples/layers/widgets/spinning_square.dart
+++ b/examples/layers/widgets/spinning_square.dart
@@ -4,7 +4,7 @@
 
 import 'package:flutter/widgets.dart';
 
-class SpinningSquare extends StatefulComponent {
+class SpinningSquare extends StatefulWidget {
   _SpinningSquareState createState() => new _SpinningSquareState();
 }