This change helps highlight certain misoptimizations with Closure
compiler. It is also stylistically preferable to consistently use index
access on index sig types.
Roughly, when one sees '.foo' they know it is always checked for typos
in the prop name by the type system (unless 'any'), while "['foo']" is
always not.
Once all angular repos are conforming this will become a tsetse.info
check, enforced by bazel.
PR Close#28937
03d2e5cb1d