In this particular case, I simply don’t want/ care to know. Finding out which dependency does not have any benefit. As long as the issue-causing syntax is part of the ES spec, the babel-preset-env will be able to transpile it at the bundle level. If anything, the untranspiled code is easier to minify using Babily.
However, to answer your question, the way I first approached this issue is by simply requiring all dependencies in one long index file and then commenting-out dependency-by-dependency until PhantomJS started to render “Hello, World!” example.