Polymer + Webpack + Karma config

karma.conf.js

const webpackConfig = require('./webpack.config.js');
const webpack = require('webpack');
const path = require('path');

delete webpackConfig.entry;
webpackConfig.bail = false;
webpackConfig.stats = 'errors-only';
webpackConfig.plugins.push(new webpack.SourceMapDevToolPlugin({
  filename: null,
  test: /\.(ts|js)($|\?)/i
}));
webpackConfig.module.rules.push({
  enforce: 'post',
  test: /\.ts$/,
  include: path.resolve(__dirname, 'src'),
  exclude: path.resolve(__dirname, 'src/test'),
  loader: 'istanbul-instrumenter-loader',
  options: {
    esModules: true
  }
});

module.exports = function(config) {
  config.set({
    basePath: '',
    browsers: ['ChromeHeadless'],
    frameworks: ['mocha'],
    reporters: ['progress', 'coverage-istanbul'],
    files: [
      { pattern: 'bower_components/webcomponentsjs/webcomponents-lite.js', included: true, watched: false },
      { pattern: 'src/test/index.ts', included: true, watched: false },
      { pattern: 'bower_components/**/*', included: false, watched: false }
    ],
    mime: {
      'text/x-typescript': ['ts']
    },
    preprocessors: {
      'src/test/index.ts': ['webpack', 'sourcemap']
    },
    webpack: webpackConfig,
    coverageIstanbulReporter: {
      dir: 'coverage/',
      reports: ['html'],
      fixWebpackSourcePaths: true
    }
  });
};