message: Use `mkdirp` so that it would work on Windows too.
diff: @@ -25,8 +25,8 @@ const fs = require('fs');
  const getPackages = require('./_getPackages');
  const glob = require('glob');
  const micromatch = require('micromatch');
  +const mkdirp = require('mkdirp');
  const path = require('path');
  -const spawnSync = require('child_process').spawnSync;
  const SRC_DIR = 'src';
  const JS_FILES_PATTERN = '**/*.js';
  @@ -69,7 +69,7 @@ function buildFile(file, silent) {
  const relativeToSrcPath = path.relative(packageSrcPath, file);
  const destPath = path.resolve(packageBuildPath, relativeToSrcPath);
  - spawnSync('mkdir', ['-p', path.dirname(destPath)]);
  + mkdirp.sync(path.dirname(destPath));
  if (micromatch.isMatch(file, IGNORE_PATTERN)) {
  silent || process.stdout.write(
  chalk.dim(' \u2022 ') +
  @@ -88,7 +88,6 @@ function buildFile(file, silent) {
  );
  } else {
  const transformed = babel.transformFileSync(file, babelOptions).code;
  - spawnSync('mkdir', ['-p', path.dirname(destPath)]);
  fs.writeFileSync(destPath, transformed);
  silent || process.stdout.write(
  chalk.green(' \u2022 ') +
  