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                // Create the production build and print the deployment instructions.
function build(previousFileSizes) {
  console.log('Creating an optimized production build...');

  let compiler = webpack(config);
  return new Promise((resolve, reject) => {, stats) => {
      if (err) {
        return reject(err);
      const messages = formatWebpackMessages(stats.toJson({}, true));
      if (messages.errors.length) {
        // Only keep the first error. Others are often indicative
        // of the same problem, but confuse the reader with noise.
        if (messages.errors.length > 1) {
          messages.errors.length = 1;
        return reject(new Error(messages.errors.join('\n\n')));
      if (
        process.env.CI &&
        (typeof process.env.CI !== 'string' ||
          process.env.CI.toLowerCase() !== 'false') &&
      ) {
            '\nTreating warnings as errors because process.env.CI = true.\n' +
              'Most CI servers set it automatically.\n'
        return reject(new Error(messages.warnings.join('\n\n')));
      return resolve({
        warnings: messages.warnings,