Dynamic Imports Support with Advanced Optimizations

## Is your feature request related to a problem? Please describe.
Add official support for dynamic import with advanced optimizations.

## Describe the solution you'd like
The issue preventing support currently is modules requesting a dynamic import cannot know the structure of public items in the requested module. As a result Closure Compiler will mangle references differently than in the bundle returned from dynamic import.

If the plugin could determine the shape of exported members from dynamic imports it could create an extern automatically for the importer to prevent mangling of public items.