Closure Compiler also has pluggability, however the plugins must be written in Java, which is a downside for the JS ecosystem. For example, asset packaging wise, there's stuff like GSS Compiler integration, where goog.getCssName('foo') is translated into an obfuscated short className, while the GSS compiler optimized, prunes, and combines GSS.
GWT was the same way, asset packaging was a core feature even in 2009. For example, it would automatically compile CSS, UI templates, and images, auto-optimizing them, and auto-sprite-sheeting everything, while packaging things into as few HTTP requests as possible.
The real issue is that to do this in a safe way for JS requires coding conventions be adhered to, or the adoption of TypeScript and annotations. Closure's advanced mode will break JS that tries to get too fancy with dynamism.