Fix internal issues caught by TS 4.9

TS 4.9 understands the `in` operator and catches two issues for us which
earlier versions did not:

1. We were checking `name in model` in `Route#serialize` without
   checking that `model` there is an object. Given that a route's model
   is *not* guaranteed to be an object, this was a runtime error waiting
   to happen. `'a' in 2` will produce `TypeError: 2 is not an Object.'

2. We were checking for `Symbol.iterator in Array` in an internal
   `iterate()` utility in the `@ember/debug/data-adapter` package. This
   check is *always* true when targeting ES6 or newer for arrays, which
   would *appear* to makie the `else` branch a no-op on all supported
   browsers. Unfortunately, at least some consumers of this API
   implement a narrower contract for iterables (presumably due to
   legacy needs to interop with IE11 in Ember < 4.x). In those cases,
   we really have an iterable, *not* an array.

packages/@ember/debug/data-adapter.ts

@@ -10,6 +10,7 @@ import { A as emberA } from '@ember/array';
 import type { Cache } from '@glimmer/validator';
 import { consumeTag, createCache, getValue, tagFor, untrack } from '@glimmer/validator';
 import type ContainerDebugAdapter from '@ember/debug/container-debug-adapter';
+import { assert } from '.';
 
 /**
 @module @ember/debug/data-adapter
@@ -34,13 +35,27 @@ type WrappedRecord<T> = {
 
 type RecordCallback<T> = (records: Array<{ columnValues: object; object: T }>) => void;
 
+// Represents the base contract for iterables as understood in the GLimmer VM
+// historically. This is *not* the public API for it, because there *is* no
+// public API for it. Recent versions of Glimmer simply use `Symbol.iterator`,
+// but some older consumers still use this basic shape.
+interface GlimmerIterable<T> {
+  length: number;
+  forEach(fn: (value: T) => void): void;
+}
+
 function iterate<T>(arr: Array<T>, fn: (value: T) => void): void {
   if (Symbol.iterator in arr) {
     for (let item of arr) {
       fn(item);
     }
   } else {
-    arr.forEach(fn);
+    // SAFETY: this cast required to work this way to interop between TS 4.8
+    // and 4.9. When we drop support for 4.8, it will narrow correctly via the
+    // use of the `in` operator above. (Preferably we will solve this by just
+    // switching to require `Symbol.iterator` instead.)
+    assert('', typeof (arr as unknown as GlimmerIterable<T>).forEach === 'function');
+    (arr as unknown as GlimmerIterable<T>).forEach(fn);
   }
 }
 

packages/@ember/routing/route.ts

@@ -353,7 +353,7 @@ class Route<T = unknown>
     if (params.length === 1) {
       let [name] = params;
       assert('has name', name);
-      if (name in model) {
+      if (typeof model === 'object' && name in model) {
         object[name] = get(model, name);
       } else if (/_id$/.test(name)) {
         object[name] = get(model, 'id');