refactor(core): simplify array flatten logic (#48358)

We can now use modern Javascript to get the same result.

PR Close #48358

packages/core/src/di/create_injector.ts

@@ -6,7 +6,6 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {flatten} from '../util/array_utils';
 import {EMPTY_ARRAY} from '../util/empty';
 import {stringify} from '../util/stringify';
 

packages/core/src/util/array_utils.ts

@@ -32,27 +32,11 @@ export function arrayEquals<T>(a: T[], b: T[], identityAccessor?: (value: T) =>
   return true;
 }
 
-
 /**
  * Flattens an array.
  */
-export function flatten(list: any[], dst?: any[]): any[] {
-  if (dst === undefined) dst = list;
-  for (let i = 0; i < list.length; i++) {
-    let item = list[i];
-    if (Array.isArray(item)) {
-      // we need to inline it.
-      if (dst === list) {
-        // Our assumption that the list was already flat was wrong and
-        // we need to clone flat since we need to write to it.
-        dst = list.slice(0, i);
-      }
-      flatten(item, dst);
-    } else if (dst !== list) {
-      dst.push(item);
-    }
-  }
-  return dst;
+export function flatten(list: any[]): any[] {
+  return list.flat(Number.POSITIVE_INFINITY);
 }
 
 export function deepForEach<T>(input: (T|any[])[], fn: (value: T) => void): void {

packages/core/test/bundling/router/bundle.golden_symbols.json

@@ -1016,9 +1016,6 @@
   {
     "name": "first"
   },
-  {
-    "name": "flatten"
-  },
   {
     "name": "flatten2"
   },