Add permitted duplicate test for shorthand properties

test/language/expressions/object/__proto__-permitted-dup-shorthand.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2020 Alexey Shvayka. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-object-initializer
+description: Permitted duplicate `__proto__` property (shorthand)
+info: |
+    Annex B defines an early error for duplicate PropertyName of `__proto__`,
+    but this does not apply to properties created from other productions.
+
+    B.3.1 __proto__ Property Names in Object Initializers
+
+    It is a Syntax Error if PropertyNameList of PropertyDefinitionList contains
+    any duplicate entries for "__proto__" and at least two of those entries
+    were obtained from productions of the form
+    PropertyDefinition : PropertyName : AssignmentExpression .
+---*/
+
+var __proto__ = 2;
+var obj = {
+  __proto__,
+  __proto__,
+};
+
+assert(obj.hasOwnProperty("__proto__"));
+assert.sameValue(obj.__proto__, 2);