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

Closes #15533
Closes #15541

BREAKING CHANGE:

Previously, `angular.isArray()` was an alias for `Array.isArray()`.
Therefore, objects that prototypally inherit from `Array` where not
considered arrays. Now such objects are considered arrays too.

This change affects several other methods that use `angular.isArray()`
under the hood, such as `angular.copy()`, `angular.equals()`,
`angular.forEach()`, and `angular.merge()`.

This in turn affects how dirty checking treats objects that prototypally
inherit from `Array` (e.g. MobX observable arrays). AngularJS will now
be able to handle these objects better when copying or watching.

src/Angular.js

@@ -219,8 +219,7 @@ 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');
 
 }
 
@@ -635,12 +634,14 @@ 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 Array.isArray(arr) || arr instanceof Array;
+}
 
 /**
  * @description

test/AngularSpec.js

@@ -1254,6 +1254,37 @@ describe('angular', function() {
     });
   });
 
+  describe('isArray', function() {
+
+    it('should return true if passed an `Array`', function() {
+      expect(isArray([])).toBe(true);
+    });
+
+    it('should return true if passed an `Array` from a different window context', function() {
+      var iframe = document.createElement('iframe');
+      document.body.appendChild(iframe);  // No `contentWindow` if not attached to the DOM.
+      var arr = new iframe.contentWindow.Array();
+      document.body.removeChild(iframe);  // Clean up.
+
+      expect(arr instanceof Array).toBe(false);
+      expect(isArray(arr)).toBe(true);
+    });
+
+    it('should return true if passed an object prototypically inherited from `Array`', function() {
+      function FooArray() {}
+      FooArray.prototype = [];
+
+      expect(isArray(new FooArray())).toBe(true);
+    });
+
+    it('should return false if passed non-array objects', function() {
+      expect(isArray(document.body.childNodes)).toBe(false);
+      expect(isArray({length: 0})).toBe(false);
+      expect(isArray({length: 2, 0: 'one', 1: 'two'})).toBe(false);
+    });
+
+  });
+
   describe('isArrayLike', function() {
 
     it('should return false if passed a number', function() {