All web components suffer from this, native or via library.
Please refer to https://github.com/slimjs/slim-webpack-boilerplate. The babel config file uses class properties plug-in and in index.html the web components polyfill is loaded.
Unfortunately with babel transpiled to es5 you must use the polyfill.

If you are addressing modern browsers that support custom elements without polyfill, then you may want to ditch babel. Consider working with rollup targeted as es6 modules.

Thanks for responding @eavichay .

For anyone else who comes across this, the polyfill did not resolve this specific problem (although it will be needed for older browsers). I was getting the above error in browsers that support the web components specification.

In order to use the latest slim-js in a project using @babel/preset-env, I needed to include it in the code transpiled through Babel and specifically call out the configuration in the rule options:

webpack.config.js:

      // module.rules
      {
        test: /\.js$/,
        exclude: /(node_modules(?!\/slim-js\/))/,
        use: {
          loader: 'babel-loader',
          options: {
            configFile: path.join(__dirname, '.babelrc')
          }
        }
      }

Since this fixes the eerror, I'll close this issue. Thanks again! 😄

If you wish to start fresh PWA project based on slim-js, with typescript support (optional), sass/HTML/image loading via import, type npm init slimjs myProjectFolder