[BUGFIX lts] Ensure inheritable observers on object proxies are strin…

…g based

Fixes #17964

packages/@ember/-internals/runtime/lib/mixins/-proxy.js

@@ -18,14 +18,6 @@ import {
 import { setProxy } from '@ember/-internals/utils';
 import { assert } from '@ember/debug';
 
-function contentPropertyDidChange(content, contentKey) {
-  let key = contentKey.slice(8); // remove "content."
-  if (key in this) {
-    return;
-  } // if shadowed in proxy
-  notifyPropertyChange(this, key);
-}
-
 export function contentFor(proxy, m) {
   let content = get(proxy, 'content');
   let tag = (m === undefined ? meta(proxy) : m).readableTag();
@@ -72,12 +64,20 @@ export default Mixin.create({
 
   willWatchProperty(key) {
     let contentKey = `content.${key}`;
-    addObserver(this, contentKey, null, contentPropertyDidChange);
+    addObserver(this, contentKey, null, '_contentPropertyDidChange');
   },
 
   didUnwatchProperty(key) {
     let contentKey = `content.${key}`;
-    removeObserver(this, contentKey, null, contentPropertyDidChange);
+    removeObserver(this, contentKey, null, '_contentPropertyDidChange');
+  },
+
+  _contentPropertyDidChange(content, contentKey) {
+    let key = contentKey.slice(8); // remove "content."
+    if (key in this) {
+      return;
+    } // if shadowed in proxy
+    notifyPropertyChange(this, key);
   },
 
   unknownProperty(key) {

packages/@ember/-internals/runtime/tests/system/object_proxy_test.js

@@ -1,6 +1,7 @@
 import { DEBUG } from '@glimmer/env';
 import {
   addObserver,
+  observer,
   computed,
   get,
   set,
@@ -317,5 +318,15 @@ moduleFor(
         'sets the `undefined` value to the proxied content'
       );
     }
+
+    ['@test should not throw or deprecate when adding an observer to an ObjectProxy based class'](
+      assert
+    ) {
+      assert.expect(0);
+
+      ObjectProxy.extend({
+        observe: observer('foo', function() {}),
+      }).create();
+    }
   }
 );