Use interior mutability for MouseEvent.
Use internal mutability for Document::idmap.
Pass immutable self to Node::{SetNodeValue, SetTextContent}.
Pass immutable document to parse_html.
Cleanup a mutable variable in dispatch_event.
Pass immutable self to HTMLBodyElement::SetOnunload.
auto merge of #2640 : Ms2ger/servo/derefmut, r=jdm