Added missing DOMNode class import

[michalbundyra]

It is used in that class but it's not imported nor defined in that namespace

• Are you fixing a bug?
  • Detail how the bug is invoked currently.
  • Detail the original, incorrect behavior.
  • Detail the new, expected behavior.
  • Base your feature on the master branch, and submit against that branch.
  • Add a regression test that demonstrates the bug, and proves the fix.
  • Add a CHANGELOG.md entry for the fix.

[weierophinney]

Thanks, @webimpress!