[BUG] Webpack tree-shaking isn't working for root imports.

Overview

Webpack tree-shaking at least as of 3.10 just isn't working is the short answer. I have a new experiment repository up at: https://github.com/FormidableLabs/rlr-tree-shaking-experiment

This mean a couple of things:

Any builds importing like import { something } from 'redux-little-router' is likely getting a lot more code and maybe even other dependencies than they should.
This explains the behavior in https://github.com/FormidableLabs/redux-little-router/issues/261 that shouldn't be happening.

The present workaround to all of this is one-off imports, which means something like instead of:

import { routerForBrowser } from 'redux-little-router';

do:

import routerForBrowser from 'redux-little-router/es/environment/browser-router';

in your frontend code and:

import routerForBrowser from 'redux-little-router/lib/environment/browser-router';

in your node code.

This is problematic for universal code which instead should probably do something like:

import routerForBrowser from 'redux-little-router/environment/browser-router';

in universal code and something like:

const path = require('path');

alias: {
  'redux-little-router': path.dirname(require.resolve('redux-little-router/es/index'))
}

in webpack config.

[ ] Figure out if there is something in this project's code that can fix things up (?)
[ ] Look to steps for specific issues like #261 such as moving at least immutable things to src/immutable/index.js instead of src/index.js

/cc @tptee