I was pretty sure I've seen a tool or a script that does this, but after spending a few minutes searching, I couldn't find it now. Hopefully someone else can recall, find, or create one.

It's not very hard to write a script, as the minifized .js file has one line per imported package.

For non-GopherJS size analysis, I've used goweight.

Thanks, I also had the same feeling yet found nothing, but decided to ask before I try to write something myself :)

After some experimentation, I was able to instrument GopherJS compiler to obtain necessary size information and generate an SVG TreeMap diagram out of it, which works both with minified and non-minified modes.

Here's an example diagram for github.com/gopherjs/gopherjs.github.io/playground:

Unfortunately, doing it as a completely standalone tool proven impossible since it needs some information that only exists at GopherJS compile time, so I'll have to submit a PR. Do maintainers have any preferences on introducing external dependencies? I have a fair bunch of code responsible for TreeMap diagram generation, which I could either put in my own repo, or make it a package in GopherJS codebase.

Meanwhile I still have some tests and cleanups to do while I'm awaiting the answer. If there's no answer, I'll assume putting that code into an external package is preferable.