Make verifiedLocale work differently depending on what it's verifying for

Make verifiedLocale work differently depending on what it's verifying for
Committed: https://code.google.com/p/dart/source/detail?r=13484

[Alan Knight, 2012-10-09 22:40:46]

This changes verifiedLocale to look at the appropriate set of locales for each
purpose, so dates are different from numbers are different from messages. It
also changes messages to use verifiedLocale, so it will now canonicalize and
look up alternate forms.

This also required making verifiedLocale able to perform a user-specified action
rather than always throwing an exception if nothing was found for a locale, so
that messages can fall back to printing the original.

There are also changes to make the example and its test need to use an alternate
form (messages specified as "de_DE", but message text is available only for
"de". Finally it changes withLocale to be able to use an arbitrary function and
return an arbitrary type. It was previously assuming it was going to be passed a
message function directly, which would only work for zero-argument message
functions. Also provided an example of using withLocale in the DartDoc comments.

[Emily Fortuna, 2012-10-09 23:37:55]

lgtm with typo fix.

+danielro@google.com on the cc list: The part here that's relevant to you, is if
you have translated messages that are only for the more general locale "en"
instead of en_US and en_GB, then you can create a file whose name is
messages_en.dart and the library will fall back to this for the general case, if
more specific localization info is not available.

https://codereview.chromium.org/11086033/diff/1/pkg/intl/lib/intl.dart
File pkg/intl/lib/intl.dart (right):

https://codereview.chromium.org/11086033/diff/1/pkg/intl/lib/intl.dart#newcode43
pkg/intl/lib/intl.dart:43: *      today(date) => Intl.message(
good catch.

https://codereview.chromium.org/11086033/diff/1/pkg/intl/lib/intl.dart#newcod...
pkg/intl/lib/intl.dart:227: * it can be a wrapper around a message function that
takes aerguments, or it
typo: arguments