title: Can AWS CDK load a pre-existing stack using an ARN? i.e. const stack = Stack.fromArn(...);
body: <p>Goal:
  I need to create an AWS ManagedPolicy that contains ALLOW permissions for API actions on resources created in a pre existing stack.  No I cannot modify the existing stack template and simply add a policy to it.  I need to create a new stack that deploys a policy that enables actions on the existing stacks resources</p>
  <p>Solution:
  Create a CDK project to generate and deploy this policy stack.  Within this CDK project I want to load the existing stack and iterate over its resources adding permissions to my new stack's policy.</p>
  <p>Problem:
  I don't see any way to load an existing stack in CDK.  I was hunting around for a &quot;Stack.fromArn(...)&quot; but don't see anything even similar.</p>
  <p>Question:
  Is there some obsucre way to do this?  Or is it simply not supported?</p>