In a range loop, the presence of a value implies the presence of a key.
However, the presence of a value as an *ast.Ident does not imply that
the key is also an *ast.Ident, thus leading to a panic any time the
two argument form is used where the key is not an identifier.

R=golang-dev, adg, r
CC=golang-dev
https://golang.org/cl/6540045