Make the argument to dispatch_event_with_target non-optional.

The name of the method makes it clear it's supposed to be used with a target
override, so we might as well enforce that.
Simplify some code in ScriptTask::load.
auto merge of #4443 : Ms2ger/servo/dispatch_event_with_target, r=Manishearth

The name of the method makes it clear it's supposed to be used with a target
override, so we might as well enforce that.
Update wptrunner and start running reftests.
Enable html/rendering.
auto merge of #4422 : Ms2ger/servo/wpt-reftests, r=larsbergstrom
auto merge of #4444 : Ms2ger/servo/load-simplify, r=jdm
Create a replacement for Cell<SM primitive>. Fixes #4337.
auto merge of #4401 : jdm/servo/heapsm2, r=Manishearth

We still will need a lint to ban any further uses, but that can be a followup.