Add appengine/gcloud/mustache dependencies.

pkg/appengine/README.md

+This package provide support for running Dart on
+[Google App Engine Managed VMs][managed-vms].
+
+### Getting started
+
+Visit [dartlang.org/cloud](https://www.dartlang.org/cloud) for more information
+on the requirements for getting started.
+
+When you are up and running a simple hello world application looks like this:
+
+```
+import 'dart:io';
+import 'package:appengine/appengine.dart';
+
+void requestHandler(HttpRequest request) {
+  request.response
+      ..write('Hello, world!');
+      ..close();
+}
+
+void main() {
+  runAppEngine(requestHandler).then((_) {
+    // Server running.
+  });
+}
+```
+
+Add the application configuration in a `app.yaml` file and run it locally using
+by running:
+
+    gcloud preview app run app.yaml
+
+When you are ready to deploy your application, make sure you have authenticated
+with `gcloud` and defined your current project. Then run:
+
+    gcloud preview app deploy app.yaml
+
+### Send Feedback
+
+We'd love to hear from you! If you encounter a bug, have suggestions for our
+APIs or is missing a feature, file it an issue on the
+[GitHub issue tracker](https://github.com/dart-lang/appengine/issues/new).
+
+**Note** The Dart support for App Engine is currently in beta.
+
+[managed-vms]: https://developers.google.com/appengine/docs/managed-vms/