A DESCRIPTION OF THE REQUEST :
Jigsaw will disallow access to internal packages, then it would be useful to create a public API allowing to obtain an indenting XMLStreamWriter.

Currently some XML API [1,2] provide public API for indenting, but StAX don't. Having public API always would be better.

[1]: http://docs.oracle.com/javase/8/docs/api/javax/xml/bind/Marshaller.html#JAXB_FORMATTED_OUTPUT
[2]: http://docs.oracle.com/javase/8/docs/api/javax/xml/transform/OutputKeys.html#INDENT

JUSTIFICATION :
Using internal API for this feature is an usual solution (when not copying data and avoiding an external dependency for only this small feature).

http://stackoverflow.com/questions/290326/stax-xml-formatting-in-java
http://stackoverflow.com/questions/4616383/xmlstreamwriter-indentation

Jigsaw will remove access to internal classes disallowing this feature. Having a public API providing equivalent API is useful for upgrading applications.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The simplest solution would be adding a new constant INDENT used when setting properties on the factory given existing method:

String indentStep = " ";
javax.xml.stream.XMLOutputFactory.setProperty(INDENT, indentStep);

(indentStep == null) would mean no indentation like currently.
(indentStep != null) would provide an instance of indenting XMLStreamWriter with setIndentStep set to the value.

Given getIndentStep/setIndentStep is currently the only specific part of com.sun.xml.txw2.output.IndentingXMLStreamWriter, all the used feature will be public.

It can be imagined another possible future enhancement: the choice of end-of-line (currently harcoded to UNIX \n). Allowing choice of end-of-line would probably require a second constant:
javax.xml.stream.XMLOutputFactory.setProperty(INDENT_EOL, "\n");

If value types exists in this time-frame, the two constants can possibly be merged in only one, setting the two properties in one step.