Prevent iOS from improperly JIT-ing isArrayLike

Fixes #2081. @jdalton found a much more efficient solution than
`'length' in collection` by using dynamic property access. Uses
a constant `LENGTH` variable to avoid minifiers transforming it
into dot access.

Author: jridgewell

underscore.js

@@ -125,9 +125,10 @@
   // Helper for collection methods to determine whether a collection
   // should be iterated as an array or as an object
   // Related: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-tolength
-  var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1;
+  // Avoids a very nasty iOS 8 JIT bug on ARM-64. #2094
+  var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1, LENGTH = 'length';
   var isArrayLike = function(collection) {
-    var length = collection != null && collection.length;
+    var length = collection != null && collection[LENGTH];
     return typeof length == 'number' && length >= 0 && length <= MAX_ARRAY_INDEX;
   };