Error: cannot find module when testing with mocha

I'm using babeljs to write an RPG engine library. I have two files:

dice.js

import assert from 'assert';
import Random from 'random-js';

export default class Dice {
  constructor(maxNumber) {
    assert(typeof(maxNumber) === "number", "maxNumber must be a number");

    this._mt = Random.engines.mt19937();
    this.minNumber = 1;
    this.maxNumber = maxNumber;
  }

  makeThrow() {
    this._mt.autoSeed();
    return Random.integer(this.minNumber, this.maxNumber)(this._mt);
  }
}

throwManager.js

import assert from 'assert';
import Dice from 'dice';
export default class ThrowManager {
  constructor(settings) {
    assert(settings.hasOwnProperty("numberOfDices"), "must set 'numberOfDices'");
    assert(settings.hasOwnProperty("maxNumberInDice"), "must set 'maxNumberInDice'");
    assert(settings.maxNumberInDice <= 1, "must have at least 1 dice");
    this.settings = settings;
  }
  execute() {
    var throwResults = [];
    for (var d = 1; d <= this.settings.numberOfDices; d++) {
      var dice = new Dice(this.settings.maxNumberInDice);
      throwResults.push(dice.makeThrow());
    };
    return throwResults;
  }
}

When I test them with mocha, I do these imports:

tests.js

var assert = require('assert');
var Amzhen = require('../Amzhen.js');
var random = require('random-js');
//tests here...

Yet when I run the tests, I get this:

Error: Cannot find module 'dice'
    at Function.Module._resolveFilename (module.js:338:15)
    at Function.Module._load (module.js:280:25)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at Object.<anonymous> (/home/joel/http://ift.tt/1Bmdhp5)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at Object.<anonymous> (/home/joel/http://ift.tt/1CROc8A)

Any ideas why the dice module isn't being found?

I'm compiling the code with babel src --out-file Amzhen.js && mocha