Better parsing of erroneous expression on left of :=. Better parsing of erroneous non-type in type switch.
Better parsing of erroneous expression on left of :=.
Better parsing of erroneous non-type in type switch.

R=iant
CC=gofrontend-dev
https://golang.org/cl/5125041

qskycolor/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,golang/gofrontend,golang/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,golang/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,golang/gofrontend