feat(isArray): make angular.isArray support Array subclasses

In particular, this change makes Angular play well with MobX observable arrays. See angular#15533

Author: thorn0

src/Angular.js

@@ -232,8 +232,7 @@ function isArrayLike(obj) {
 
   // NodeList objects (with `item` method) and
   // other objects with suitable length characteristics are array-like
-  return isNumber(length) &&
-    (length >= 0 && ((length - 1) in obj || obj instanceof Array) || typeof obj.item === 'function');
+  return isNumber(length) && (length >= 0 && (length - 1) in obj || typeof obj.item === 'function');
 
 }
 
@@ -634,12 +633,14 @@ function isDate(value) {
  * @kind function
  *
  * @description
- * Determines if a reference is an `Array`. Alias of Array.isArray.
+ * Determines if a reference is an `Array`.
  *
  * @param {*} value Reference to check.
  * @returns {boolean} True if `value` is an `Array`.
  */
-var isArray = Array.isArray;
+function isArray(arr) {
+  return arr instanceof Array || Array.isArray(arr);
+}
 
 /**
  * @ngdoc function

test/AngularSpec.js

@@ -1226,6 +1226,16 @@ describe('angular', function() {
     });
   });
 
+  describe('isArray', function() {
+
+    it('should return true if passed an object prototypically inherited from Array.prototype', function() {
+      function FooArray() {}
+      FooArray.prototype = [];
+      expect(isArray(new FooArray())).toBe(true);
+    });
+
+  });
+
   describe('isArrayLike', function() {
 
     it('should return false if passed a number', function() {