chore(import): don't monkey patch ID but use `attrs` utility

lib/import/BpmnImporter.js

@@ -26,6 +26,13 @@ import {
 } from './Util';
 
 
+/**
+ * @param {ModdleElement} semantic
+ * @param {ModdleElement} di
+ * @param {Object} [attrs=null]
+ *
+ * @return {Object}
+ */
 function elementData(semantic, di, attrs) {
   return assign({
     id: semantic.id,
@@ -105,13 +112,12 @@ BpmnImporter.prototype.add = function(semantic, di, parentElement) {
   // invisible root element (process, subprocess or collaboration)
   if (is(di, 'bpmndi:BPMNPlane')) {
 
-    // add a virtual element (not being drawn)
-    element = this._elementFactory.createRoot(elementData(semantic, di));
+    var attrs = is(semantic, 'bpmn:SubProcess')
+      ? { id: semantic.id + '_plane' }
+      : {};
 
-    // for subprocesses, the id is already defined on the collapsed shape
-    if (is(semantic, 'bpmn:SubProcess')) {
-      element.id = element.id + '_plane';
-    }
+    // add a virtual element (not being drawn)
+    element = this._elementFactory.createRoot(elementData(semantic, di, attrs));
 
     this._canvas.addRootElement(element);
   }