Object.observe: notify when element addition causes array growth

test/mjsunit/harmony/object-observe.js

Index: test/mjsunit/harmony/object-observe.js
diff --git a/test/mjsunit/harmony/object-observe.js b/test/mjsunit/harmony/object-observe.js
index 231fe91e18594797c5519eec6fbc9f2b839998ef..d4fb48bfe7904618846105e39d85d6c4d35c4b3c 100644
--- a/test/mjsunit/harmony/object-observe.js
+++ b/test/mjsunit/harmony/object-observe.js
@@ -399,3 +399,19 @@ observer.assertCallbackRecords([
   { object: obj, name: "3", type: "new" },
   { object: obj, name: "4", type: "new" },
 ]);
+
+// Adding elements past the end of an array should notify on length
+reset();
+var arr = [1, 2, 3];
+Object.observe(arr, observer.callback);
+arr[3] = 10;
+arr[100] = 20;
+arr[50] = 30; // no length change expected
+Object.deliverChangeRecords(observer.callback);
+observer.assertCallbackRecords([
+  { object: arr, name: 'length', type: 'updated', oldValue: 3 },
+  { object: arr, name: '3', type: 'new' },
+  { object: arr, name: 'length', type: 'updated', oldValue: 4 },
+  { object: arr, name: '100', type: 'new' },
+  { object: arr, name: '50', type: 'new' },
+]);