Fix freeze scope extension warning

Fixes #455 (for now)

Also add a comment about how what we're doing here is unsafe (for reasons that
are mostly unrelated to the warning)
auto merge of #656 : kmcallister/servo/dynamic-freeze, r=jdm

Fixes #455 (for now)

Also add a comment about how what we're doing here is unsafe (for reasons that are mostly unrelated to the warning)

r? @tkuehn