go/parser: accept optional indices for all parts of an index expression

Instead, leave the error testing to the type checker, eventually.

Fixes issue 5827.

R=adonovan
CC=golang-dev
https://codereview.appspot.com/10917043
doc: update playground.js

R=dsymonds
CC=golang-dev
https://codereview.appspot.com/10933044
Merge branch 'master' into gitfix