Fix image_dynamic_remove reftest with incremental layout turned out

This also adds some extra debugging infrastructure which I found useful tracking
this bug down. A regression in the br reftests is also uncovered by this patch,
which I'll work on fixing next.

r? @pcwalton
Fixed the inline_element_border_a reftest with incremental layout turned on.
auto merge of #3706 : cgaebel/servo/fix-image-dynamic-remove, r=pcwalton

This also adds some extra debugging infrastructure which I found useful tracking
this bug down. A regression in the br reftests is also uncovered by this patch,
which I'll work on fixing next.

EDIT: nevermind. no regression, I just tested that before a rebase.

r? @pcwalton
Wait to dump flow tree until flow is complete

The flow tree is currently dumped for debugging purposes early on in
the flow process, so many values are still zero. If we wait to dump it
until later, the output will more accurately reflect the real flow tree.
auto merge of #3719 : mrobinson/servo/flow-dump, r=pcwalton

The flow tree is currently dumped for debugging purposes early on in
the flow process, so many values are still zero. If we wait to dump it
until later, the output will more accurately reflect the real flow tree.
Edits dom/bindings/DESIGN.md

This corrects and improves the dom/bindings/DESIGN.md document and also
corrects a grammatical error in a tests/reftest.rs message.
auto merge of #3720 : gilles-leblanc/servo/fix-grammatical-errors, r=jdm

This corrects and improves the dom/bindings/DESIGN.md document and also
corrects a grammatical error in a tests/reftest.rs message.
Don't crash on invalid utf-8 in the HTML parser.

This was regressed by the html5ever landing.