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.
In other words, CLJS users don't have to do anything special to benefit from the power of Google Closure. It just works :)
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.