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CSS map changes doesn't apply, if module was changed.
It's due all modules in node js exposed as cached objects.
All reference saved in `require.cache`.

This tiny helper, that we use for clear `require.cache`:

```
  /**
   * Clears node.js require cache by key.
   * @param {string} key Cache key.
   */
  function clearCacheKey (key) {
    delete require.cache[key];
  }
```
