Currently the source code of the catalog is not minified. This appears to be removed by this commit. It is however unclear what the code was breaking.
Personally I have used minification techniques such as uglifyjs and htmlminifier with success and not breaking my application. Since this is one of the official Polymer sources, it is used as sample application by new users to see how Polymer is used in the wild and we got some questions in the Polymer Slack regarding the unoptimized source code of the catalog.
Currently the source code of the catalog is not minified. This appears to be removed by this commit. It is however unclear what the code was breaking.
Personally I have used minification techniques such as uglifyjs and htmlminifier with success and not breaking my application. Since this is one of the official Polymer sources, it is used as sample application by new users to see how Polymer is used in the wild and we got some questions in the Polymer Slack regarding the unoptimized source code of the catalog.