I have the patch ready. It needs changes in xml/core as well, as the xml module does not honor the contract defined by 
org.openide.loaders.XMLDataObject that allows to assign different node to each public ID.

#103946: Providing a node for layer.xml and modifying xml/core to honor the 'node delegate' contract defined by 
org.openide.loaders.XMLDataObject 

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Is there any particular reason why we could not simply define a MIME resolver for e.g. "text/x-netbeans-layer+xml"
according to the DTD and make a trivial DataObject type for layers? This is, after all, what we recommend, do in all
other circumstances that I am aware of, and even provide a template for in apisupport! I don't follow why we are using
this strange old trick with the DataNode cookie, which seems like a relict from an otherwise unused set of
XMLDataObject-related APIs. I would suggest that we deprecate Environment.Provider, not try to find new uses for it.

I do not think you can deprecate it. Lookup at xml/lookups and it seems to be used pretty much.

Re. apisupport template - I was just thinking why we do not support Env.Prov based node in our wizards today. It is 
established and well working API (except the problem with xml/core which ignored it in 6.0) and is generally more 
efficient than introducing new loader.

However if you do not like my changes in apisupport, feel free to revert them. Just keep the test in openide/loaders 
and code in xml/core, imho.

According to

http://deadlock.netbeans.org/hudson/job/trunk/lastSuccessfulBuild/artifact/nbbuild/build/generated/layers.txt

except for the usage you just introduced, all usages of xml/lookups seem to be for settings registration (in our old
pre-Preferences style), not for user-level files. All other user files that I know of have a proper MIME type and a loader.

Definitely we should not add a wizard to generate code using a technique no one uses for that purpose, when there is
already a wizard that uses the normal technique, and the normal technique is more powerful (since arbitrary MIME
resolution algorithms beyond lookup by DTD are possible). This would be very confusing.

A new loader adds a bit of overhead today. pnejedly claims this will be fixed soon.

Of course the feature of the recognition of layer files is appreciated, no matter how it is implemented. I would not
want to revert it. If I have time to change it to use the normal style I probably will.