Google Closure has been successfully tree-shaking since it was released, in 2009. ClojureScript uses Closure for an optimization path, so the majority of CLJS apps in production (CircleCI, Prismatic, to name a few) use tree-shaking on every deploy.
One can still write code that breaks the tree-shaking. How does Google Closure solve that? Through its community and best practices. Arguably, this is where Smalltalk failed, and where other dynamic languages can take a useful lesson. (Being fostered by an environment like Google probably gave Closure a leg up this way.)
ClojureScript uses Closure for an optimization path
ClojureScript has an advantage, in that it's probably designed to not-easily produce code that breaks tree shaking.
The entire JS toolchain lost five to ten years by not adopting Closure. I suspect that in a year or two, someone will release a webpack plugin that does the exact same thing Closure did, with the exact same constraints.
In other words, CLJS users don't have to do anything special to benefit from the power of Google Closure. It just works :)