ERROR when trying to install the polymer-cli

If you try to install the polymer-cli:

$ sudo npm install -g polymer-cli

...and you get the following error:

Error: EACCES: permission denied, mkdir '/usr/lib/node_modules/polymer-cli/node_modules/wd/build'

You can solve it by running the command with the --unsafe-perm parameter:

$ sudo npm install -g polymer-cli --unsafe-perm