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

Closes #15533

BREAKING CHANGE:

`angular.isArray` is used by `angular.copy`, which in turn is used internally by the dirty
checking logic. That's why this change affects the way objects are copied and watched by AngularJS.
Objects that prototypally inherit from `Array` (e.g. MobX observable arrays, see #15533) weren't
previously recognized as arrays, now they are. This change also affects `angular.merge`,
`angular.forEach`, and `angular.equals`.

Previously, `angular.isArray` was an alias for and thus worked exactly as `Array.isArray`.

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 arr instanceof Array || Array.isArray(arr);
+}
 
 /**
  * @description

test/AngularSpec.js

@@ -1251,6 +1251,16 @@ 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() {