Work around Rust bug causing nondeterministic CSS parse failure

The bug is mozilla/rust#10683 and there's no fix yet, plus it
would take us a while to upgrade Rust anyway.

Fixes #1258.
auto merge of #1328 : kmcallister/servo/properties, r=jdm

The bug is mozilla/rust#10683 and there's no fix yet, plus it would take us a while to upgrade Rust anyway.

Fixes #1258.