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

pkg/intl/lib/src/date_format_internal.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.
 
 /**
  * This contains internal implementation details of the date formatting code
  * which are exposed as public functions because they must be called by other
  * libraries in order to configure the source for the locale data. We don't want
  * them exposed as public API functions in the date formatting library, so they
  * are put in a separate library here. These are for internal use only. User
  * code should import one of the `date_symbol_data...` libraries and call the
  * `initializeDateFormatting` method exposed there.
  */
 
 library date_format_internal;
 import 'intl_helpers.dart';
+import '../date_symbols.dart';
 
 /**
  * This holds the symbols to be used for date/time formatting, indexed
  * by locale. Note that it will be set differently during initialization,
  * depending on what implementation we are using. By default, it is initialized
  * to an instance of UninitializedLocaleData, so any attempt to use it will
  * result in an informative error message.
  */
 var dateTimeSymbols =
-    const UninitializedLocaleData('initializeDateFormatting(<locale>)');
+    new UninitializedLocaleData('initializeDateFormatting(<locale>)',
+        en_USSymbols);
 
 /**
  * This holds the patterns used for date/time formatting, indexed
  * by locale. Note that it will be set differently during initialization,
  * depending on what implementation we are using. By default, it is initialized
  * to an instance of UninitializedLocaleData, so any attempt to use it will
  * result in an informative error message.
  */
 var dateTimePatterns =
-    const UninitializedLocaleData('initializeDateFormatting(<locale>)');
+    new UninitializedLocaleData('initializeDateFormatting(<locale>)',
+        en_USPatterns);
 
 /**
  * Initialize the symbols dictionary. This should be passed a function that
  * creates and returns the symbol data. We take a function so that if
  * initializing the data is an expensive operation it need only be done once,
  * no matter how many times this method is called.
  */
 void initializeDateSymbols(Function symbols) {
   if (dateTimeSymbols is UninitializedLocaleData) {
     dateTimeSymbols = symbols();
   }
 }
 
 /**
  * Initialize the patterns dictionary. This should be passed a function that
  * creates and returns the pattern data. We take a function so that if
  * initializing the data is an expensive operation it need only be done once,
  * no matter how many times this method is called.
  */
 void initializeDatePatterns(Function patterns) {
   if (dateTimePatterns is UninitializedLocaleData) {
     dateTimePatterns = patterns();
   }
 }
 
 Future initializeIndividualLocaleDateFormatting(Function init) {
   return init(dateTimeSymbols, dateTimePatterns);
 }