Added MergeableMaterial Widget. (#5165)

MergeableMaterial is an animated container that knows how to merge
separate slices of Material together.

packages/flutter/lib/material.dart

@@ -50,6 +50,7 @@ export 'src/material/input.dart';
 export 'src/material/list.dart';
 export 'src/material/list_item.dart';
 export 'src/material/material.dart';
+export 'src/material/mergeable_material.dart';
 export 'src/material/overscroll_indicator.dart';
 export 'src/material/page.dart';
 export 'src/material/paginated_data_table.dart';