astutil: in test, dump AST before and after on failure
astutil: in test, dump AST before and after on failure

R=golang-dev, crawshaw
CC=golang-dev
https://codereview.appspot.com/37740043

chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,lattera/go.tools,lattera/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,lattera/go.tools