[BUGFIX beta] Ensures Ember.A is a constructor

The recent change to transpilation has made `new A()` break, because
A is an arrow function and arrow functions cannot be constructors. This
is a pretty common pattern in Ember apps and likely constitutes a
breaking change, so this PR fixes it by changing the arrow function to
a normal function.

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

@@ -1641,9 +1641,11 @@ let A;
 
 if (ENV.EXTEND_PROTOTYPES.Array) {
   NativeArray.apply(Array.prototype);
-  A = arr => arr || [];
+  A = function(arr) {
+    return arr || [];
+  };
 } else {
-  A = arr => {
+  A = function(arr) {
     if (!arr) {
       arr = [];
     }