Implement "global" property of global object

test/mjsunit/harmony/global.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: --harmony-global
+
+function Test() {
+  var globalObject = global;
+  assertEquals(globalObject, global);
+  assertEquals(globalObject, global.global);
+
+  var thisObject = this;
+  assertEquals(globalObject, thisObject);
+  assertEquals(globalObject, thisObject.global);
+
+  const desc = Object.getOwnPropertyDescriptor(globalObject, 'global');
+  assertPropertiesEqual({
+    value: globalObject,
+    writable: true,
+    enumerable: false,
+    configurable: true
+  }, desc);
+}
+Test();