Document date format initialization better and make it possible to use en_US dates without it

pkg/intl/lib/date_symbols.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library date_symbols;
 
 /**
  * This holds onto information about how a particular locale formats dates. It
  * contains mostly strings, e.g. what the names of months or weekdays are,
  * but also indicates things like the first day of the week. We expect the data
  * for instances of these to be generated out of ICU or a similar reference
@@ skipping 94 matching lines @@
     map["DATEFORMATS"] = DATEFORMATS;
     map["TIMEFORMATS"] = TIMEFORMATS;
     map["AVAILABLEFORMATS"] = AVAILABLEFORMATS;
     map["FIRSTDAYOFWEEK"] = FIRSTDAYOFWEEK;
     map["WEEKENDRANGE"] = WEEKENDRANGE;
     map["FIRSTWEEKCUTOFFDAY"] = FIRSTWEEKCUTOFFDAY;
     return map;
   }
 
   toString() => NAME;
-}
+}
+
+/**
+ * We hard-code the locale data for en_US here so that there's at least one
+ * locale always available.
+ */
+var en_USSymbols = new DateSymbols(
+    NAME: "en_US",
+    ERAS: const [ 'BC', 'AD'],
+    ERANAMES: const [ 'Before Christ', 'Anno Domini'],
+    NARROWMONTHS: const [ 'J', 'F', 'M', 'A', 'M', 'J', 'J', 'A', 'S', 'O',
+         'N', 'D'],
+    STANDALONENARROWMONTHS: const [ 'J', 'F', 'M', 'A', 'M', 'J', 'J', 'A',
+         'S', 'O', 'N', 'D'],
+    MONTHS: const [ 'January', 'February', 'March', 'April', 'May', 'June',
+         'July', 'August', 'September', 'October', 'November', 'December'],
+    STANDALONEMONTHS: const [ 'January', 'February', 'March', 'April', 'May',
+         'June', 'July', 'August', 'September', 'October', 'November',
+         'December'],
+    SHORTMONTHS: const [ 'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul',
+         'Aug', 'Sep', 'Oct', 'Nov', 'Dec'],
+    STANDALONESHORTMONTHS: const [ 'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
+         'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'],
+    WEEKDAYS: const [ 'Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday',
+         'Friday', 'Saturday'],
+    STANDALONEWEEKDAYS: const [ 'Sunday', 'Monday', 'Tuesday', 'Wednesday',
+         'Thursday', 'Friday', 'Saturday'],
+    SHORTWEEKDAYS: const [ 'Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'],
+    STANDALONESHORTWEEKDAYS: const [ 'Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri',
+         'Sat'],
+    NARROWWEEKDAYS: const [ 'S', 'M', 'T', 'W', 'T', 'F', 'S'],
+    STANDALONENARROWWEEKDAYS: const [ 'S', 'M', 'T', 'W', 'T', 'F', 'S'],
+    SHORTQUARTERS: const [ 'Q1', 'Q2', 'Q3', 'Q4'],
+    QUARTERS: const [ '1st quarter', '2nd quarter', '3rd quarter',
+         '4th quarter'],
+    AMPMS: const [ 'AM', 'PM'],
+    DATEFORMATS: const [ 'EEEE, MMMM d, y', 'MMMM d, y', 'MMM d, y',
+         'M/d/yy'],
+    TIMEFORMATS: const [ 'h:mm:ss a zzzz', 'h:mm:ss a z', 'h:mm:ss a',
+         'h:mm a'],
+    FIRSTDAYOFWEEK: 6,
+    WEEKENDRANGE: const [5, 6],
+    FIRSTWEEKCUTOFFDAY: 5);
+
+var en_USPatterns = const {
+  'd': 'd', // DAY
+  'E': 'EEE', // ABBR_WEEKDAY
+  'EEEE': 'EEEE', // WEEKDAY
+  'LLL': 'LLL', // ABBR_STANDALONE_MONTH
+  'LLLL': 'LLLL', // STANDALONE_MONTH
+  'M': 'L', // NUM_MONTH
+  'Md': 'M/d', // NUM_MONTH_DAY
+  'MEd': 'EEE, M/d', // NUM_MONTH_WEEKDAY_DAY
+  'MMM': 'LLL', // ABBR_MONTH
+  'MMMd': 'MMM d', // ABBR_MONTH_DAY
+  'MMMEd': 'EEE, MMM d', // ABBR_MONTH_WEEKDAY_DAY
+  'MMMM': 'LLLL', // MONTH
+  'MMMMd': 'MMMM d', // MONTH_DAY
+  'MMMMEEEEd': 'EEEE, MMMM d', // MONTH_WEEKDAY_DAY
+  'QQQ': 'QQQ', // ABBR_QUARTER
+  'QQQQ': 'QQQQ', // QUARTER
+  'y': 'y', // YEAR
+  'yM': 'M/y', // YEAR_NUM_MONTH
+  'yMd': 'M/d/y', // YEAR_NUM_MONTH_DAY
+  'yMEd': 'EEE, M/d/y', // YEAR_NUM_MONTH_WEEKDAY_DAY
+  'yMMM': 'MMM y', // YEAR_ABBR_MONTH
+  'yMMMd': 'MMM d, y', // YEAR_ABBR_MONTH_DAY
+  'yMMMEd': 'EEE, MMM d, y', // YEAR_ABBR_MONTH_WEEKDAY_DAY
+  'yMMMM': 'MMMM y', // YEAR_MONTH
+  'yMMMMd': 'MMMM d, y', // YEAR_MONTH_DAY
+  'yMMMMEEEEd': 'EEEE, MMMM d, y', // YEAR_MONTH_WEEKDAY_DAY
+  'yQQQ': 'QQQ y', // YEAR_ABBR_QUARTER
+  'yQQQQ': 'QQQQ y', // YEAR_QUARTER
+  'H': 'HH', // HOUR24
+  'Hm': 'HH:mm', // HOUR24_MINUTE
+  'Hms': 'HH:mm:ss', // HOUR24_MINUTE_SECOND
+  'j': 'h a', // HOUR
+  'jm': 'h:mm a', // HOUR_MINUTE
+  'jms': 'h:mm:ss a', // HOUR_MINUTE_SECOND
+  'jmv': 'h:mm a v', // HOUR_MINUTE_GENERIC_TZ
+  'jmz': 'h:mm a z', // HOUR_MINUTETZ
+  'jz': 'h a z', // HOURGENERIC_TZ
+  'm': 'm', // MINUTE
+  'ms': 'mm:ss', // MINUTE_SECOND
+  's': 's', // SECOND
+  'v': 'v', // ABBR_GENERIC_TZ
+  'z': 'z', // ABBR_SPECIFIC_TZ
+  'zzzz': 'zzzz', // SPECIFIC_TZ
+  'ZZZZ': 'ZZZZ'  // ABBR_UTC_TZ
+};