I've tested this on Microsoft Edge and Chrome and its working.
Update: It's working now on Firefox
I have the same issue currently. This is a compatibility issue with Firefox version 64.0. Previous version 63.0.3 works with Google Cloud Shell but version 64.0 released on 2018-12-11 shows Cloud Shell empty.
You can use other browser or fix it temporarily by reverting back to older Firefox version from https://support.mozilla.org/en-US/kb/install-older-version-of-firefox.
There was a problem with Google Cloud Shell terminal emulator, but it has since been fixed.
来源：https://stackoverflow.com/questions/53812905/cloud-terminal-appears-empty-in-firefox