[BUGFIX] Don't setup mandatory setters on array indexes

It turns out that removing items from an array (e.g. via `splice`) will
also remove their descriptors, if any. Could be a browser bug, but I'm
not sure we intended to add setters directly to arrays in the first
place, and if we can't definitely manage them we probably shouldn't.

(cherry picked from commit 5aa4c61)

packages/@ember/-internals/runtime/tests/system/object/create_test.js

@@ -1,5 +1,5 @@
 import { getOwner, setOwner } from '@ember/-internals/owner';
-import { computed, Mixin, observer } from '@ember/-internals/metal';
+import { computed, Mixin, observer, addObserver, destroy } from '@ember/-internals/metal';
 import { DEBUG } from '@glimmer/env';
 import EmberObject from '../../../lib/system/object';
 import { moduleFor, AbstractTestCase } from 'internal-test-helpers';
@@ -51,6 +51,43 @@ moduleFor(
       }
     }
 
+    ['@test does not sets up separate mandatory setters on getters'](assert) {
+      if (DEBUG) {
+        let MyClass = EmberObject.extend({
+          get foo() {
+            return 'bar';
+          },
+          fooDidChange: observer('foo', function() {}),
+        });
+
+        let o = MyClass.create({});
+        assert.equal(o.get('foo'), 'bar');
+
+        let descriptor = Object.getOwnPropertyDescriptor(o, 'foo');
+        assert.ok(!descriptor, 'Mandatory setter was not setup');
+
+        // cleanup
+        o.destroy();
+      } else {
+        assert.expect(0);
+      }
+    }
+
+    ['@test does not sets up separate mandatory setters on arrays'](assert) {
+      if (DEBUG) {
+        let arr = [123];
+
+        addObserver(arr, 0, function() {});
+
+        let descriptor = Object.getOwnPropertyDescriptor(arr, 0);
+        assert.ok(!descriptor.set, 'Mandatory setter was not setup');
+
+        destroy(arr);
+      } else {
+        assert.expect(0);
+      }
+    }
+
     ['@test calls setUnknownProperty if defined'](assert) {
       let setUnknownPropertyCalled = false;
 

packages/@ember/-internals/utils/lib/mandatory-setter.ts

@@ -14,6 +14,19 @@ export let setWithMandatorySetter:
 
 type PropertyDescriptorWithMeta = PropertyDescriptor & { hadOwnProperty?: boolean };
 
+function isElementKey(key: string | number | symbol) {
+  return typeof key === 'number' ? isPositiveInt(key) : isStringInt(key as string);
+}
+
+function isStringInt(str: string) {
+  let num = parseInt(str, 10);
+  return isPositiveInt(num) && str === String(num);
+}
+
+function isPositiveInt(num: number) {
+  return num >= 0 && num % 1 === 0;
+}
+
 if (DEBUG) {
   let SEEN_TAGS = new WeakSet();
 
@@ -34,6 +47,10 @@ if (DEBUG) {
 
     SEEN_TAGS!.add(tag);
 
+    if (Array.isArray(obj) && isElementKey(keyName)) {
+      return;
+    }
+
     let desc = (lookupDescriptor(obj, keyName) as PropertyDescriptorWithMeta) || {};
 
     if (desc.get || desc.set) {