babel.config.js

module.exports = {
  presets: [
    ['@babel/env', {
      loose: true,
      useBuiltIns: 'usage',
      ...process.env.NODE_ENV === 'test' ? {} : {
        modules: false,
      },
    }],
    '@babel/typescript',
  ],
  plugins: [
    ['template-html-minifier', {
      modules: {
        'lit-html': ['html', 'svg'], // lit-html
        'lit-element': ['html', 'svg'], // LitElement
        '@polymer/polymer/polymer-element': ['html'], // Polymer 3 - exported from Polymer Element
        '@polymer/polymer/lib/utils/html-tag.js': ['html'], // Polymer 3 - exported from utils (used by PolymerElements family)
      },
      htmlMinifier: {
        collapseWhitespace: true,
        removeComments: true,
        minifyCSS: true,
      },
    }],
    ['@babel/transform-runtime', {
      corejs: 2,
      sourceType: 'unambiguous',
    }],
    ['@babel/proposal-decorators', {
      decoratorsBeforeExport: true,
    }],
    ['@babel/proposal-class-properties', {
      loose: true,
    }],
    '@babel/syntax-dynamic-import',
  ],
};