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, making the
   `else` branch a no-op on all currently-supported browsers.

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

@@ -34,16 +34,6 @@ type WrappedRecord<T> = {
 
 type RecordCallback<T> = (records: Array<{ columnValues: object; object: T }>) => 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);
-  }
-}
-
 class RecordsWatcher<T> {
   recordCaches: Map<T, Cache<void>> = new Map();
 
@@ -88,10 +78,10 @@ class RecordsWatcher<T> {
       // Track `[]` for legacy support
       consumeTag(tagFor(records, '[]'));
 
-      iterate(records, (record) => {
+      for (let record of records) {
         getValue(this.getCacheForItem(record));
         seen.add(record);
-      });
+      }
 
       // Untrack this operation because these records are being removed, they
       // should not be polled again in the future
@@ -133,9 +123,10 @@ class TypeWatcher {
     let hasBeenAccessed = false;
 
     this.cache = createCache(() => {
-      // Empty iteration, we're doing this just
-      // to track changes to the records array
-      iterate(records, () => {});
+      for (let _ of records) {
+        // Empty iteration, we're doing this just
+        // to track changes to the records array
+      }
 
       // Also track `[]` for legacy support
       consumeTag(tagFor(records, '[]'));

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');