(another parameter switcharoo found by errorprone)

Implementers and callers of SerializerBase.namespaceAfterStartElement disagree about parameter names. All should be sanity checked/tested.
E.g.:

$ pwd; grep -r public.*namespaceAfterStartElement.*prefix
...jdk10/jaxp/src/java.xml/share/classes/com/sun/org/apache/xml/internal/serializer
ExtendedContentHandler.java: public void namespaceAfterStartElement(String uri, String prefix)
ToUnknownStream.java: public void namespaceAfterStartElement(String prefix, String uri)
SerializerBase.java: public void namespaceAfterStartElement(String uri, String prefix)
ToHTMLStream.java: public void namespaceAfterStartElement(String prefix, String uri)
EmptySerializer.java: public void namespaceAfterStartElement(String uri, String prefix)