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.
Some objects that weren't previously recognized as arrays (e.g. MobX observable arrays, see #15533)
by the watching logic are now recognized as such.

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

Author: thorn0

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() {