Allow resource load failures to send an error string which is reporting during task failure.
Fix for parallel data race in layout code. Fixes #2308.
Add ProgressEvent interface
auto merge of #2355 : glennw/servo/http-fail-errors, r=jdm

...during task failure.
auto merge of #2361 : Manishearth/servo/progressevent, r=jdm

Adds the ProgressEvent webidl and implementation according to the XHR spec.

Blocks #2282
auto merge of #2369 : glennw/servo/issue-2308, r=metajack

There might be a "cleaner" rust way to separate the scope and invoke the drop() call?
Suppress dead code warnings for start when building servo.rs as a dylib.
auto merge of #2371 : zwarich/servo/start-dead-code, r=metajack