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

pkg/intl/lib/src/intl_helpers.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.
 
 /**
  * A library for general helper code associated with the intl library
  * rather than confined to specific parts of it.
  */
 
 library intl_helpers;
+import '../date_symbols.dart';
 
 /**
  * This is used as a marker for a locale data map that hasn't been initialized,
- * and will throw an exception on any usage.
+ * and will throw an exception on any usage that isn't the fallback
+ * patterns/symbols provided.
  */
-class UninitializedLocaleData {
+class UninitializedLocaleData<F> {
   final String message;
-  const UninitializedLocaleData(this.message);
+  final F fallbackData;
+  const UninitializedLocaleData(this.message, this.fallbackData);
 
-  operator [](String key) {
-    _throwException();
-  }
+  operator [](String key) =>
+      (key == 'en_US') ? fallbackData : _throwException();
   List get keys => _throwException();
-  bool containsKey(String key) => _throwException();
+  bool containsKey(String key) => (key == 'en_US') ? true : _throwException();
 
   _throwException() {
     throw new LocaleDataException("Locale data has not been initialized"
         ", call $message.");
   }
 }
 
 class LocaleDataException implements Exception {
   final String message;
   LocaleDataException(this.message);
   toString() => "LocaleDataException: $message";
 }
 
 /**
  *  An abstract superclass for data readers to keep the type system happy.
  */
 abstract class LocaleDataReader {
   Future read(String locale);
 }
 
 /**
  * The internal mechanism for looking up messages. We expect this to be set
  * by the implementing package so that we're not dependent on its
  * implementation.
  */
 var messageLookup = const
-    UninitializedLocaleData('initializeMessages(<locale>)');
+    UninitializedLocaleData('initializeMessages(<locale>)', null);
 
 /**
  * Initialize the message lookup mechanism. This is for internal use only.
  * User applications should import `message_lookup_local.dart` and call
  * `initializeMessages`
  */
 void initializeInternalMessageLookup(Function lookupFunction) {
   if (messageLookup is UninitializedLocaleData) {
     messageLookup = lookupFunction();
   }
 }