Add a dateSymbols getter to DateFormat, useful to get e.g. the list of weekday names

Add a dateSymbols getter to DateFormat, useful to get e.g. the list of weekday names

R=efortuna@google.com

Committed: https://code.google.com/p/dart/source/detail?r=22749

[Alan Knight, 2013-05-14 20:34:34]

This adds a getter to DateFormat that will return the DateSymbols instance for
its locale. I had some hesitation adding this because it exposes what's
otherwise an internal object. However, there are strong use cases for wanting to
get the list of possible names without having to go through format routines to
get them, and the object is really just a list of field names that come form
ICU/CLDR, so keeping around something that exposes those field names even if we
change the implementation seems reasonable.

It's an instance method rather than static because then we know that the locale
exists, it will have gone through looking for the more general locale, and so
forth.

[Emily Fortuna, 2013-05-15 15:32:18]

lgtm

[Alan Knight, 2013-05-15 17:54:47]

Committed patchset #1 manually as r22749 (presubmit successful).