[ReactNative] Revert async exports changes to MessageQueue + test

Summary:
@public

`[Bridge] Add support for JS async functions to RCT_EXPORT_METHOD` was imported but broke some internal code, reverting the `MessageQueue` that caused the issues and add a test, since the method is not used yet.

Test Plan: Run the test o/

Examples/UIExplorer/UIExplorerIntegrationTests/js/PromiseTest.js

@@ -0,0 +1,50 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule PromiseTest
+ */
+'use strict';
+
+var RCTTestModule = require('NativeModules').TestModule;
+var React = require('react-native');
+
+var PromiseTest = React.createClass({
+
+  shouldResolve: false,
+  shouldReject: false,
+
+  componentDidMount() {
+    Promise.all([
+      this.testShouldResolve(),
+      this.testShouldReject(),
+    ]).then(() => RCTTestModule.finish(
+      this.shouldResolve && this.shouldReject
+    ));
+  },
+
+  testShouldResolve() {
+    return RCTTestModule
+      .shouldResolve()
+      .then(() => this.shouldResolve = true)
+      .catch(() => this.shouldResolve = false);
+  },
+
+  testShouldReject() {
+    return RCTTestModule
+      .shouldReject()
+      .then(() => this.shouldReject = false)
+      .catch(() => this.shouldReject = true);
+  },
+
+  render() {
+      return <React.View />;
+  }
+
+});
+
+module.exports = PromiseTest;