It looks like uglifyjs can process multiple filenames or STDIN.
So I think you have two options:
Option 1 - Pipe the contents of the file into uglifyjs
dir .\build -filter *.js | Get-Content | uglifyjs -o bundle.js
Option 2 - Pass the filenames into uglifyjs
$files = (dir .\build -filter *.js | select -expandproperty Name) -Join " "
uglifyjs $files -o bundle.js