code: function getPreimage (web3, key) {
    return new Promise((resolve, reject) => {
      web3.debug.preimage(key.indexOf('0x') === 0 ? key : '0x' + key, function (error, preimage) {
        if (error) {
          resolve(null)
        } else {
          resolve(preimage)
        }
      })
    })
  }
docs: Uses web3 to return preimage of a key
  
  @param {String} key  - key to retrieve the preimage of
  @return {String} - preimage of the given key