Revert "auto merge of #8645 : alexcrichton/rust/issue-6436-run-non-block...

...ing, r=brson"

This reverts commit b8d1fa399402c71331aefd634d710004e00b73a6, reversing
changes made to f22b4b169854c8a4ba86c16ee43327d6bcf94562.

Conflicts:
	mk/rt.mk
	src/libuv

This caused a big performance regression on the windows bots and possibly some unexpected segfaults in pretty-printing tests.