Promises: Add regression test for promise resolution with proxy

BUG=v8:5004

Review-Url: https://codereview.chromium.org/2070213002
Cr-Commit-Position: refs/heads/master@{#37015}

test/mjsunit/regress/regress-5004.js

+// Copyright 2015 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
+
+function assertAsync(b, s) {
+  if (!b) {
+    %AbortJS(" FAILED!")
+  }
+}
+
+class P extends Promise {
+  constructor() {
+    super(...arguments)
+    return new Proxy(this, {
+      get: (_, key) => {
+        return key == 'then' ?
+            this.then.bind(this) :
+            this.constructor.resolve(20)
+      }
+    })
+  }
+}
+
+let p = P.resolve(10)
+p.key.then(v => assertAsync(v === 20));