make short date formats localizable

[naomiblack]

lib/formatter/date.dart defines some short formats for dates -- these are currently hardcoded for en_US. They should be localizable.

@vicb proposes that we use the patterns defined in https://github.com/dart-lang/bleeding_edge/blob/master/dart/pkg/intl/lib/date_time_patterns.dart

[vicb]

Naomi I had already opened #1019