Renamed expansion panel file. (#5884)

Renamed expansion_panels.dart to expansion_panel.dart.

packages/flutter/lib/material.dart

@@ -35,7 +35,7 @@ export 'src/material/drawer_header.dart';
 export 'src/material/drawer_item.dart';
 export 'src/material/drop_down.dart';
 export 'src/material/expand_icon.dart';
-export 'src/material/expansion_panels.dart';
+export 'src/material/expansion_panel.dart';
 export 'src/material/flat_button.dart';
 export 'src/material/flexible_space_bar.dart';
 export 'src/material/floating_action_button.dart';