After updating to the latest version of Craft (3.0.1) the dashboard is throwing an error:
Variable "CraftClient" does not exist

Same issue here.
Craft version: 3.0.1
PHP version: 7.1.12
For anyone running into this, can you try clearing your craft/storage/runtime/compiled_templates folder and see if that helps? If not, can you check and see if you have opcache or any other PHP byte level caching extensions installed on your box like APC?
@angrybrad clearing the compiled templates folder did the trick!
Starting in the next release, Craft will make sure all compiled templates are cleared out whenever Craft or a plugin is updated, to guard against this sort of thing. (7c1b32a85652c360ef99e94bc94d6303c0aa0f91)
Most helpful comment
Starting in the next release, Craft will make sure all compiled templates are cleared out whenever Craft or a plugin is updated, to guard against this sort of thing. (7c1b32a85652c360ef99e94bc94d6303c0aa0f91)