sending testnet coins from one address to another. Problem: dont know how to generate ecKey using new bitcoinjs api

gistfile1.txt

var bitcoin = require('bitcoinjs-lib')
var helloblock = require('helloblock-js')({
  network: 'testnet',
  debug: true
})

//source btc account testnet

var sourcePrivateKeyWIF = '91fKWDmVZz7y7SyM5fLUBTuK2eUSYd7wxkwujQ7YLbNxNQU4LiW'; //balance 0.051 BTC
var sourceBTCAddress = 'migNkRq9gLmdPhgFfBBUiWaZf17uFE87yq';

// @SIDA, I think to be missing a ecKey - > what is this key? what is it used for?

//destination private keys and address
var key = bitcoin.ECKey.makeRandom();
var destPrivateKeyWIF = key.toWIF(bitcoin.networks.testnet) //private key
var destBTCAddress = key2.pub.getAddress(bitcoin.networks.testnet).toString();
console.log('dest private key: ', destPrivateKeyWIF);
console.log('dest addres: ', destBTCAddress);

console.log("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")

var txFee = 10000
var txTargetValue = 150000

helloblock.addresses.getUnspents(sourceBTCAddress, {
  value: txTargetValue + txFee
}, function(err, res, unspents) {
  if (err) throw new Error(err)

  var tx = new bitcoin.Transaction()

  var totalUnspentsValue = 0
  unspents.forEach(function(unspent) {
    tx.addInput(unspent.txHash, unspent.index)
    totalUnspentsValue += unspent.value
  })

  tx.addOutput(destBTCAddress, txTargetValue)

  var txChangeValue = totalUnspentsValue - txTargetValue - txFee
  tx.addOutput(sourceBTCAddress, txChangeValue)

  tx.ins.forEach(function(input, index) {
    tx.sign(index, ecKey) //WHAT IS THE ECKEY? how do i get it?
  })

  var rawTxHex = tx.toHex() //changed from tx.serializeHex()

  helloblock.transactions.propagate(rawTxHex, function(err, res, tx) {
    if (err) throw new Error(err)

    console.log('SUCCESS: https://test.helloblock.io/transactions/' + tx.txHash)
  })
})