Build script doesn't work with the @material/mwc-elements

[larsgk]

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).

[larsgk]

Related to material-components/material-web#257

[aomarks]

I think this is more-or-less a dupe of material-components/material-web#257, and I think will be fixed if we switch to typescript's importHelpers style of polyfills, instead of using the globally-injected style.