fix(forEach): match behaviour of Array.prototype.forEach (ignore missing properties)

Array.prototype.forEach will not invoke the callback function if the properety is not present in the
object. Because of this, we have the illusion of not iterating over non-added properties in a sparse
array.

From ECMAScript:

9. Repeat while k < len
     a. Let Pk be ToString(k).
     b. Let kPresent be HasProperty(O, Pk).
     c. ReturnIfAbrupt(kPresent).
     d. If kPresent is true, then
            i. Let kValue be Get(O, Pk)
            ... (steps for invoking the function and aborting if it throws)

Closes angular#8510
Closes angular#8522

Author: caitp

src/Angular.js

@@ -246,7 +246,9 @@ function forEach(obj, iterator, context) {
       }
     } else if (isArray(obj) || isArrayLike(obj)) {
       for (key = 0, length = obj.length; key < length; key++) {
-        iterator.call(context, obj[key], key);
+        if (key in obj) {
+          iterator.call(context, obj[key], key);
+        }
       }
     } else if (obj.forEach && obj.forEach !== forEach) {
         obj.forEach(iterator, context);

test/AngularSpec.js

@@ -650,6 +650,16 @@ describe('angular', function() {
       forEach(obj, function(value, key) { log.push(key + ':' + value); });
       expect(log).toEqual(['length:2', 'foo:bar']);
     });
+
+
+    it('should not invoke the iterator for indexed properties which are not present in the collection', function() {
+      var log = [];
+      var collection = [];
+      collection[5] = 'SPARSE';
+      forEach(collection, function (item, index) { log.push(item + index); });
+      expect(log.length).toBe(1);
+      expect(log[0]).toBe('SPARSE5');
+    });
   });