If an element is created from JS, it might have its contents modified
before it is inserted into the document. In this case, we don't have a
shadow tree yet and so trying to set m_text_node's text content would
cause a null dereference. So let's not do that. :^)
That case also means that by the time we do create the shadow tree, we
have the text content already, so we can set it there.
Added a test to verify that we don't crash, and that the text content
appears in the textarea whether it was inserted by JS or by the HTML
parser.
