code: function circularImportsError(fileBeingImported, importTrail) {
    const message =
      `Circular import of "${fileBeingImported}".\r\n` +
      'Import trace:\r\n' +
      importTrail.map(f => ` -> ${f}`).join('\r\n')
    return new Error(message)
  }
docs: Constructs a circular imports error.
  
  @param  {string} fileBeingImported
  The file being imported.
  
  @param  {string} importingFile
  The file that imported.
  
  @return {Error}
  An error.