Inline canUseNextTick and installNextTickImplementation.

A savings of 71 bytes uglified.
Inline the postMessage implementation for better minification.

A savings of 36 bytes.
Inline MessageChannel implementation for better minification.

A savings of 35 bytes.
Inline onReadyStateChange installation for better minification.

A savings of 63 bytes uglified.
Inline setTimeout installation for better minification.

A savings of 24 bytes uglified.
Get global object more reliably and using fewer bytes.

18 bytes isn't a huge savings, but this code works even when the global
variable is not already defined, in any JS environment.
Set attachTo.setImmediate only once, for better minification.

A savings of 46 bytes uglified.