By using https://babeljs.io/docs/en/babel-plugin-proposal-private-methods

bundling lit-element with a component adds about 100KB (unminified) code to the output.

I'd love to see a way to make lit-element an external, since we're planning on a bunch of npm packages with different components in it.

Since this is the example that is linked to by the Lit-Element website, I went ahead and tried to get it working with a very minimal component and refactored it to work on Windows to no avail.

I cloned this repo just to see if it would work and it runs into the same error I got on my project.

The system cannot find the file specified.
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! [email protected] copywc: `copy -r node_modules\@webcomponents\webcomponentsjs\bundles build && copy node_modules\@webcomponents\webcomponentsjs\webcomponents-loader.js build`

So I am not exactly sure how to resolve this, but the lack of documentation is frustrating to say the least. Much appreciated for putting this together.

A cross platform version that actually works would be ideal though.

Reproduce:

• Clone repo
• Update lit-element to latest (2.1.0)
• Build and verify that build works (it does ;))
• Add a component e.g. npm install @material/mwc-button & import/add in my-element
  (The element version is 0.5.0)
• Clean and build

Two things happen:

1. the build throws an error:

src/index.js → build/index.js...
(!) `this` has been rewritten to `undefined`
https://rollupjs.org/guide/en#error-this-is-undefined
node_modules/@material/mwc-button/mwc-button.js
1: var __decorate = this && this.__decorate || function (decorators, target, key, desc) {
                    ^
2:   var c = arguments.length,
3:       r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc,
...and 1 other occurrence

and 2. the app never renders - but throws an error in the console:

util.js:43 Uncaught TypeError: Cannot read property 'appendChild' of null
    at detectEdgePseudoVarBug (util.js:43)
    at supportsCssVariables (util.js:76)
    at ripple-directive.js:26

Using the rollup script from https://open-wc.org/building/building-rollup.html works with the mwc-elements.

Also, other elements done with lit-element work (with this script).

I even include custom-elements-es5-adapter.js file to index.html, still it is not working in IE11. It looks like we need to configure something in rollup.config.js with babel.