_addonTreesFor(type) {
    return this.project.addons.reduce((sum, addon) => {
      if (addon.treeFor) {
        let tree = addon.treeFor(type);
        if (tree && !mergeTrees.isEmptyTree(tree)) {
          sum.push({
            name: addon.name,
            tree,
            root: addon.root,
          });
        }
      }
      return sum;
    }, []);
  }
Loads and initializes addons for this project.
    Calls initializeAddons on the Project.

    @private
    @method initializeAddons