Completely my fault. I used knockout.js uility method ko.utils.cloneNodes despite the fact that it is not exported using ko.exportSymbol in the source code of the library.
Result? The mehtod name is obfuscated and is not accessible in minified version of the library. And I got a script error complaining of undefined method. Luckily it is just a development environment.
Must be more careful to not repeat the same error again. And there is even a special note regarding exactly the issue I stumbled upon on the knockout download page at http://knockoutjs.com/downloads/: ".... Don't use it for normal application development, because it exposes additional unsupported private APIs."