[elements] cleaning up string wrapper elements kind and adding tests

test/mjsunit/string-wrapper.js

Index: test/mjsunit/string-wrapper.js
diff --git a/test/mjsunit/string-wrapper.js b/test/mjsunit/string-wrapper.js
new file mode 100644
index 0000000000000000000000000000000000000000..2d44eaef8e482778ba5a980c69d81777d76d7d75
--- /dev/null
+++ b/test/mjsunit/string-wrapper.js
@@ -0,0 +1,41 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var limit = 10000;
+
+function testStringWrapper(string) {
+  assertEquals('a', string[0]);
+  assertEquals('b', string[1]);
+  assertEquals('c', string[2]);
+}
+
+(function testFastStringWrapperGrow() {
+  var string = new String("abc");
+  for (var i = 0; i < limit; i += 2) {
+    string[i] = {};
+  }
+  testStringWrapper(string);
+
+  for (var i = limit; i > 0; i -= 2) {
+    delete string[i];
+  }
+  testStringWrapper(string);
+})();
+
+(function testSlowStringWrapperGrow() {
+  var string = new String("abc");
+  // Force Slow String Wrapper Elements Kind
+  string[limit] = limit;
+  for (var i = 0; i < limit; i += 2) {
+    string[i] = {};
+  }
+  testStringWrapper(string);
+  assertEquals(limit, string[limit]);
+
+  for (var i = limit; i > 0; i -= 2) {
+    delete string[i];
+  }
+  testStringWrapper(string);
+  assertEquals(undefined, string[limit]);
+})();