make it obvious arguments are optional

because now they have a setter

src/editor/model.js

@@ -18,7 +18,7 @@ limitations under the License.
 import {diffAtCaret, diffDeletion} from "./diff";
 
 export default class EditorModel {
-    constructor(parts, partCreator, updateCallback) {
+    constructor(parts, partCreator, updateCallback = null) {
         this._parts = parts;
         this._partCreator = partCreator;
         this._activePartIdx = null;

src/editor/parts.js

@@ -363,7 +363,7 @@ export function autoCompleteCreator(getAutocompleterComponent, updateQuery) {
 }
 
 export class PartCreator {
-    constructor(room, client, autoCompleteCreator) {
+    constructor(room, client, autoCompleteCreator = null) {
         this._room = room;
         this._client = client;
         this._autoCompleteCreator = {create: autoCompleteCreator && autoCompleteCreator(this)};