Object.defineProperty shouldn't be a hint that we're constructing a dictionary.

test/mjsunit/regress/regress-362870.js

+// Copyright 2014 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
+
+// Adding a property via Object.defineProperty should not be taken as hint that
+// we construct a dictionary, quite the opposite.
+var obj = {};
+
+for (var i = 0; i < 100; i++) {
+  Object.defineProperty(obj, "x" + i, { value: 31415 });
+  Object.defineProperty(obj, "y" + i, {
+    get: function() { return 42; },
+    set: function(value) { }
+  });
+  assertTrue(%HasFastProperties(obj));
+}