From 6954ade6203f1d7b9dd8d83b95c0d8fa319fe58b Mon Sep 17 00:00:00 2001
From: Matthew Beale <matt.beale@madhatted.com>
Date: Sat, 17 Dec 2016 14:08:01 -0800
Subject: [PATCH] [BUGFIX] Only freeze empty array/dict with weakmap

When arrays and objects are frozen in JavaScript, it is impossible to
attach meta-data (like Ember's own `meta`) to them without using a
WeakMap. Ember does adopt the WeakMap strategy in browsers that support
it, however there are still supported environments (IE9, IE10) where
`Object.freeze` is supported but WeakMap is not. But not freezing these
empty arrays and object if WeakMap is missing, legacy meta-data
strategies are permitted on those instances.

See:

* https://github.com/emberjs/ember.js/issues/14264
* https://github.com/emberjs/ember.js/pull/14244
---
 packages/glimmer-runtime/lib/utils.ts | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/packages/glimmer-runtime/lib/utils.ts b/packages/glimmer-runtime/lib/utils.ts
index 67cb6b039b..21e310bbfc 100644
--- a/packages/glimmer-runtime/lib/utils.ts
+++ b/packages/glimmer-runtime/lib/utils.ts
@@ -1,7 +1,18 @@
 import { Dict, dict } from 'glimmer-util';
 
-export const EMPTY_ARRAY = Object.freeze([]);
-export const EMPTY_DICT: Dict<any> = Object.freeze(dict<any>());
+const HAS_NATIVE_WEAKMAP = (function() {
+  // detect if `WeakMap` is even present
+  let hasWeakMap = typeof WeakMap === 'function';
+  if (!hasWeakMap) { return false; }
+
+  let instance = new WeakMap();
+  // use `Object`'s `.toString` directly to prevent us from detecting
+  // polyfills as native weakmaps
+  return Object.prototype.toString.call(instance) === '[object WeakMap]';
+})();
+
+export const EMPTY_ARRAY = HAS_NATIVE_WEAKMAP ? Object.freeze([]) : [];
+export const EMPTY_DICT: Dict<any> = HAS_NATIVE_WEAKMAP ? Object.freeze(dict<any>()) : dict<any>();
 
 export interface EnumerableCallback<T> {
   (item: T): void;