[Reproducible-builds] Bug#794162: FTBFS: missing build-dep on uglifyjs(?)

[Chris West (Faux)]

Source: ruby-cal-heatmap-rails
Version: 0.0.1~dfsg-1
Severity: serious
Tags: sid stretch
Justification: fails to build from source
User: reproducible-builds at lists.alioth.debian.org
Usertags: ftbfs

Dear Maintainer,

The package fails to build:

dh_auto_build
	dh_ruby --build
   dh_ruby --build
uglifyjs -o vendor/assets/javascripts/cal-heatmap.min.js vendor/assets/javascripts/cal-heatmap.js
make[1]: uglifyjs: Command not found
debian/rules:21: recipe for target 'override_dh_auto_build' failed

Full build log:
https://reproducible.debian.net/rb-pkg/unstable/amd64/ruby-cal-heatmap-rails.html

-- System Information:
Debian Release: stretch/sid
  APT prefers unstable
  APT policy: (500, 'unstable')
Architecture: amd64 (x86_64)

Kernel: Linux 3.19.0-23-generic (SMP w/8 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)