go/types: typechecking conversions, part 1 (non-constants)
go/types: typechecking conversions, part 1 (non-constants)

R=adonovan
CC=golang-dev
https://codereview.appspot.com/7103058

dsriram/golang,apparentlymart/golang,gnanderson/go,gnanderson/go,apparentlymart/golang,dsriram/golang,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme