Reinstate fixes to AST walker
Reinstate fixes to AST walker

Issue #263

marijnh/acorn,mathiasbynens/acorn,Rich-Harris/acorn,mathiasbynens/acorn,marijnh/acorn,happibum/acorn,btmills/acorn,ajaxorg/acorn,mysticatea/acorn,ajaxorg/acorn,Rich-Harris/acorn,Rich-Harris/acorn,mathiasbynens/acorn,MarcelGerber/acorn,happibum/acorn,TimothyGu/acorn,MarcelGerber/acorn,mysticatea/acorn,xiemaisi/acorn,btmills/acorn,mysticatea/acorn,btmills/acorn,MarcelGerber/acorn,TimothyGu/acorn,ternjs/acorn,ternjs/acorn,marijnh/acorn,happibum/acorn,xiemaisi/acorn,TimothyGu/acorn,xiemaisi/acorn,ternjs/acorn,ajaxorg/acorn