Multiple bundles (and cache) for user-agents

In the near future (or already now), we need to handle these browsers:
- Native ES6, native module
- Native ES7 supported, but no native module
- Native ES6 supported, but no native module
- No native ES6, no native module

Already we have no need to ES6 transpile on Chrome and Safari 10, but we still do that. That could be big overhead for such the latest browsers. I think we should make different bundle for each environment.

No files, tests, or entry points are named. Start by locating the bundler's user-agent handling and current ES6 transpilation path; define the supported browser groups and verify that each receives an appropriate bundle without unnecessary transpilation.

Written by the indexing model from the issue text.