I wrote a test for CodeHolder>>spawnHierarchy. It's neat, it works... 
and it opens up a HierarchyBrowser. I have a reference to the 
HierarchyBrowser but not the SystemWindow that contains it.

What I'm trying to find is what to put here:

spawn := browser spawnHierarchy.
["run my tests"]
     ensure: ["what goes here to close the SystemWindow?"]

I had thought of this:

PluggableSystemWindow allInstances
     detect: [ :each | each model == spawn ]
     ifFound: [ :each | each delete ]
     ifNone: []]

which works if you make CodeHolder>>spawnHierarchy return the new 
HierarchyBrowser.

Is this a simply dreadful idea? Is there a less hacky way than this? 
Would this work outside Morphic?

frank