If no pattern is specified for date formatting, use a default

pkg/intl/lib/date_format.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.
 
 /**
  * DateFormat is for formatting and parsing dates in a locale-sensitive
  * manner.
  * It allows the user to choose from a set of standard date time formats as well
  * as specify a customized pattern under certain locales. Date elements that
  * vary across locales include month name, week name, field order, etc.
@@ skipping 408 matching lines @@
    * that are used to do the actual formatting and parsing. Do not use
    * this variable directly, use the getter [_formatFields].
    */
   List<_DateFormatField> _formatFieldsPrivate;
 
   /**
    * Getter for [_formatFieldsPrivate] that lazily initializes it.
    */
   get _formatFields {
     if (_formatFieldsPrivate == null) {
+      if (_pattern == null) _useDefaultPattern();
       _formatFieldsPrivate = parsePattern(_pattern);
     }
     return _formatFieldsPrivate;
   }
 
   /**
+   * We are being asked to do formatting without having set any pattern.
+   * Use a default.
+   */
+  _useDefaultPattern() {
+    add_yMMMMd();
+    add_jms();
+  }
+
+  /**
    * A series of regular expressions used to parse a format string into its
    * component fields.
    */
   static var _matchers = const [
       // Quoted String - anything between single quotes, with escaping
       //   of single quotes by doubling them.
       // e.g. in the pattern "hh 'o''clock'" will match 'o''clock'
       const RegExp("^\'(?:[^\']|\'\')*\'"),
       // Fields - any sequence of 1 or more of the same field characters.
       // e.g. in "hh:mm:ss" will match hh, mm, and ss. But in "hms" would
       // match each letter individually.
       const RegExp(
         "^(?:G+|y+|M+|k+|S+|E+|a+|h+|K+|H+|c+|L+|Q+|d+|m+|s+|v+|z+|Z+)"),
       // Everything else - A sequence that is not quotes or field characters.
       // e.g. in "hh:mm:ss" will match the colons.
       const RegExp("^[^\'GyMkSEahKHcLQdmsvzZ]+")
   ];
 
   /**
    * Set our pattern, appending it to any existing patterns. Also adds a single
    * space to separate the two.
    */
-  _setPattern(String inputPattern, [String separator = ' ']) {
+  _appendPattern(String inputPattern, [String separator = ' ']) {
     if (_pattern == null) {
       _pattern = inputPattern;
     } else {
       _pattern = "$_pattern$separator$inputPattern";
     }
   }
 
   /**
    * Add [inputPattern] to this instance as a pattern. If there was a previous
    * pattern, then this appends to it, separating the two by [separator].
    * [inputPattern] is first looked up in our list of known skeletons.
    * If it's found there, then use the corresponding pattern for this locale.
    * If it's not, then treat [inputPattern] as an explicit pattern.
    */
   DateFormat addPattern(String inputPattern, [String separator = ' ']) {
     // TODO(alanknight): This is an expensive operation. Caching recently used
     // formats, or possibly introducing an entire "locale" object that would
     // cache patterns for that locale could be a good optimization.
-    if (!_availableSkeletons.containsKey(inputPattern)) {
-      _setPattern(inputPattern, separator);
-    } else {
-      _setPattern(_availableSkeletons[inputPattern], separator);
-    }
     // If we have already parsed the format fields, reset them.
     _formatFieldsPrivate = null;
+    if (inputPattern == null) return this;
+    if (!_availableSkeletons.containsKey(inputPattern)) {
+      _appendPattern(inputPattern, separator);
+    } else {
+      _appendPattern(_availableSkeletons[inputPattern], separator);
+    }
     return this;
   }
 
   /** Return the pattern that we use to format dates.*/
   get pattern => _pattern;
 
   /** Return the skeletons for our current locale. */
   Map get _availableSkeletons {
     return dateTimePatterns[locale];
   }
@@ skipping 58 matching lines @@
   List _reverse(List list) {
     // TODO(alanknight): Use standardized list reverse when implemented.
     // See Issue 2804.
     var result = new List();
     for (var i = list.length-1; i >= 0; i--) {
       result.addLast(list[i]);
     }
     return result;
   }
 }