[tiler] Add some more <T>s

TESTED_ON=device

Change-Id: If8e8007f75d9d9a8b5aa427235c520a00c4c34a8
diff --git a/lib/tiler/lib/src/tile.dart b/lib/tiler/lib/src/tile.dart
index 257572c..42ab07c 100644
--- a/lib/tiler/lib/src/tile.dart
+++ b/lib/tiler/lib/src/tile.dart
@@ -132,7 +132,7 @@
   TileModel parent;
   TileType type;
   T content;
-  List<TileModel> tiles;
+  List<TileModel<T>> tiles;
 
   TileModel({
     @required this.type,
@@ -141,7 +141,7 @@
     this.tiles,
     double flex = 1,
   }) : _flex = flex {
-    tiles ??= <TileModel>[];
+    tiles ??= <TileModel<T>>[];
   }
 
   // Defines the flex factor on how the tile is sized.