Fix an accidentally quadratic reduce(concat) in FlattenedNodesObserver (

#5739)

Found by @mvanbem-goog debugging a perf issue in Gerrit

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Change Log
 
+## Unreleased
+- FlattenedNodesObserver.getFlattenedNodes was accidentally quadratic if given a non-slot node. Found by @mvanbem-goog.
+
 ## [v3.5.1](https://github.com/Polymer/polymer/tree/v3.5.1) (2022-06-03)
 - [ci skip] bump to 3.5.1 ([commit](https://github.com/Polymer/polymer/commit/2cbb3d2b))
 

lib/utils/flattened-nodes-observer.js

@@ -88,14 +88,17 @@ export let FlattenedNodesObserver = class {
       node = /** @type {!HTMLSlotElement} */(node); // eslint-disable-line no-self-assign
       return wrapped.assignedNodes({flatten: true});
     } else {
-      return Array.from(wrapped.childNodes).map((node) => {
+      const results = [];
+      for (let i = 0; i < wrapped.childNodes.length; i++) {
+        const node = wrapped.childNodes[i];
         if (isSlot(node)) {
-          node = /** @type {!HTMLSlotElement} */(node); // eslint-disable-line no-self-assign
-          return wrap(node).assignedNodes({flatten: true});
+          const slotNode = /** @type {!HTMLSlotElement} */ (node);
+          results.push(...wrap(slotNode).assignedNodes({ flatten: true }));
         } else {
-          return [node];
+          results.push(node);
         }
-      }).reduce((a, b) => a.concat(b), []);
+      }
+      return results;
     }
   }
 
@@ -311,4 +314,4 @@ export let FlattenedNodesObserver = class {
     }
   }
 
-};
+};