Modify app/BUILD: bower_components to fetch map files too

In preparation for Polymer 2.x update, as with the webcomponents update,
it pulls in a .map file.

Change-Id: I8c12afbd98780ead07089a6c38e4b7c223d87ea1

polygerrit-ui/app/BUILD

@@ -51,6 +51,7 @@ filegroup(
         [
             "bower_components/**/*.html",
             "bower_components/**/*.js",
+            "bower_components/**/*.map",
         ],
     ),
 )