Reland of [stubs] KeyedStoreGeneric: inline dictionary property stores

test/mjsunit/regress/regress-private-enumerable.js

 // 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.
 
+// Flags: --allow-natives-syntax
+
 class A {}
 class B {}
 Object.assign(B, A);
 assertEquals("class B {}", B.toString());
+
+(function() {
+  function f(a, i, v) {
+    a[i] = v;
+  }
+
+  f("make it generic", 0, 0);
+
+  var o = {foo: "foo"};
+  %OptimizeObjectForAddingMultipleProperties(o, 10);
+
+  var s = %CreatePrivateSymbol("priv");
+  f(o, s, "private");
+  %ToFastProperties(o);
+
+  var desc = Object.getOwnPropertyDescriptor(o, s);
+  assertEquals("private", desc.value);
+  assertTrue(desc.writable);
+  assertFalse(desc.enumerable);
+  assertTrue(desc.configurable);
+})();