Make PropertyDeclarationBlock have Arcs.
Ensure that it's safe to modify the DOM node pointers before doing so. Fixes #1224.
Add forgotten test for Element.attributes.
auto merge of #1252 : jdm/servo/layoutassert, r=jdm,metajack

...224.
auto merge of #1311 : pradeep90/servo/rule-hash-optimize, r=pcwalton

Commit for #1301
auto merge of #1326 : brunoabinader/servo/rename-flowcontext, r=pcwalton

This patch is for:
https://github.com/mozilla/servo/issues/1282
Remove 'FloatFlow'

Removes 'FloatFlow' in favor of FloatBlockFlow, which is cointained
inside BlockFlow in a 'has-a' relationship. This avoids a bunch of
duplicated code.

This patch is for:
https://github.com/mozilla/servo/issues/1281