[async-iteration] expose Symbol.asyncIterator behind flag --harmony-async-iteration

test/mjsunit/harmony/symbol-async-iterator.js

Index: test/mjsunit/harmony/symbol-async-iterator.js
diff --git a/test/mjsunit/harmony/symbol-async-iterator.js b/test/mjsunit/harmony/symbol-async-iterator.js
new file mode 100644
index 0000000000000000000000000000000000000000..8a92add6354b1e1e29d42729a80b9122bc024188
--- /dev/null
+++ b/test/mjsunit/harmony/symbol-async-iterator.js
@@ -0,0 +1,14 @@
+// Copyright 2017 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-async-iteration
+
+assertTrue(Symbol.hasOwnProperty('asyncIterator'));
+assertEquals('symbol', typeof Symbol.asyncIterator);
+assertInstanceof(Object(Symbol.asyncIterator), Symbol);
+
+let desc = Object.getOwnPropertyDescriptor(Symbol, 'asyncIterator');
+assertFalse(desc.writable);
+assertFalse(desc.enumerable);
+assertFalse(desc.configurable);