Make callers of `${Derived}::${base}()` use casts instead.
Remove UIEvent.event().
Remove HTMLElement.element().
Remove Node.eventtarget().
Remove Element.node().
Fixes #4131

A slight refactor.
Remove default value for params in mach doc #4134
auto merge of #4135 : murphm8/servo/mach_doc, r=SimonSapin

This should fix  #4134
auto merge of #4130 : saneyuki/servo/cast, r=Manishearth

Fix #4124

This also introduce `BarCast::from_actual()` which is used for up-cast for dom's actual data types (non JS pointer values).
auto merge of #4132 : jtuchsen/servo/issue-4131, r=jdm

A slight refactor.
Shared Sniffer Task

- Added TargetedLoadResponse and ResponseSenders
- LoadData constructor contains the next consumer which means
  SnifferManager doesn't need the next consumer to start
- New SnifferTask is created at new resource_task creation
- Update Unit Tests
auto merge of #4000 : t29/servo/mime-sniffing, r=jdm

Issue: #3144

This PR addresses the second step of the ticket. i.e. move from a 1:1 sniffer:request task model to a shared sniffer task.