addon/helpers/storage.js
function storageFor(key, modelName, options = {}) {
  if (arguments.length === 2 && typeof modelName === 'object') {
    options = modelName;
    modelName = null;
  }

  assert('The options argument must be an object', typeof options === 'object');

  // normalize key
  key = dasherize(key);

  if (!modelName) {
    return computed(function() {
      if (!storages[key]) {
        storages[key] = createStorage(this, key, null, options);
      }

      return storages[key];
    });
  }

  assert('The second argument must be a string', typeof modelName === 'string');

  return computed(modelName, function() {
    const model = this.get(modelName);

    // if the propertyValue is null/undefined we simply return null/undefined
    if (!model || typeof model === 'undefined') {
      return model;
    }

    const modelKey = _modelKey(model);
    const storageKey = `${key}:${modelKey}`;
    // TODO allow callbacks to delete the storage if model gets deleted

    if (!storages[storageKey]) {
      storages[storageKey] = createStorage(this, key, modelKey, options);
    }

    return storages[storageKey];
  });
}
function storageFor(key, modelName, options = {}) {
  if (arguments.length === 2 && typeof modelName === 'object') {
    options = modelName;
    modelName = null;
  }

  assert('The options argument must be an object', typeof options === 'object');

  // normalize key
  key = dasherize(key);

  if (!modelName) {
    return computed(function() {
      if (!storages[key]) {
        storages[key] = createStorage(this, key, null, options);
      }

      return storages[key];
    });
  }

  assert('The second argument must be a string', typeof modelName === 'string');

  return computed(modelName, function() {
    const model = this.get(modelName);

    // if the propertyValue is null/undefined we simply return null/undefined
    if (!model || typeof model === 'undefined') {
      return model;
    }

    const modelKey = _modelKey(model);
    const storageKey = `${key}:${modelKey}`;
    // TODO allow callbacks to delete the storage if model gets deleted

    if (!storages[storageKey]) {
      storages[storageKey] = createStorage(this, key, modelKey, options);
    }

    return storages[storageKey];
  });
}
TODO: v2.0 - Remove options