Remove synthetic unresolved variables from async/await desugaring

test/mjsunit/regress/regress-5405.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-async-await --allow-natives-syntax
+
+let log = [];
+
+(async function() {
+  with ({get ['.promise']() { log.push('async') }}) {
+    return 10;
+  }
+})();
+%RunMicrotasks();
+
+(function() {
+  with ({get ['.new.target']() { log.push('new.target') }}) {
+    return new.target;
+  }
+})();
+
+(function() {
+  with ({get ['this']() { log.push('this') }}) {
+    return this;
+  }
+})();
+
+// TODO(v8:5405): The log should actually be empty and not
+// contain new.target.
+assertArrayEquals(['new.target'], log);