Add @nocollapse for jscompiler

See google/closure-compiler#2763

lib/utils/flattened-nodes-observer.js

@@ -76,7 +76,8 @@ export class FlattenedNodesObserver {
    *
    * @param {HTMLElement|HTMLSlotElement} node The node for which to return the list of flattened nodes.
    * @return {Array} The list of flattened nodes for the given `node`.
-  */
+   * @nocollapse See https://github.com/google/closure-compiler/issues/2763
+   */
   static getFlattenedNodes(node) {
     if (isSlot(node)) {
       node = /** @type {HTMLSlotElement} */(node); // eslint-disable-line no-self-assign
@@ -97,7 +98,7 @@ export class FlattenedNodesObserver {
    * @param {Element} target Node on which to listen for changes.
    * @param {?function(!Element, { target: !Element, addedNodes: !Array<!Element>, removedNodes: !Array<!Element> }):void} callback Function called when there are additions
    * or removals from the target's list of flattened nodes.
-  */
+   */
   constructor(target, callback) {
     /**
      * @type {MutationObserver}