[BUGFIX beta] [FEATURE EMBER_GLIMMER_ARRAY_HELPER] Add feature flagging.

This feature was accidentally landed without feature flagging (:sob:),
this commit adds feature flagging (defaulting to off unless manually
enabled).

packages/@ember/-internals/glimmer/lib/resolver.ts

@@ -6,6 +6,7 @@ import {
   EMBER_MODULE_UNIFICATION,
   GLIMMER_CUSTOM_COMPONENT_MANAGER,
   GLIMMER_MODIFIER_MANAGER,
+  EMBER_GLIMMER_ARRAY_HELPER,
 } from '@ember/canary-features';
 import { assert } from '@ember/debug';
 import { _instrumentStart } from '@ember/instrumentation';
@@ -66,7 +67,6 @@ const BUILTINS_HELPERS = {
   concat,
   get,
   hash,
-  array,
   log,
   mut,
   'query-params': queryParams,
@@ -84,6 +84,10 @@ const BUILTINS_HELPERS = {
   '-assert-implicit-component-helper-argument': componentAssertionHelper,
 };
 
+if (EMBER_GLIMMER_ARRAY_HELPER) {
+  BUILTINS_HELPERS['array'] = array;
+}
+
 const BUILTIN_MODIFIERS = {
   action: { manager: new ActionModifierManager(), state: null },
 };