migrate raw string to new syntax in pkg

pkg/intl/lib/src/date_format_field.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 is a private class internal to DateFormat which is used for formatting
  * particular fields in a template. e.g. if the format is hh:mm:ss then the
  * fields would be "hh", ":", "mm", ":", and "ss". Each type of field knows
  * how to format that portion of a date.
  */
@@ skipping 71 matching lines @@
 
   parse(_Stream input, _DateBuilder dateFields) {
     return parseLiteral(input);
   }
 
   void patchQuotes() {
      if (pattern == "''") {
       pattern = "'";
     } else {
       pattern = pattern.substring(1, pattern.length - 1);
-      var twoEscapedQuotes = new RegExp(@"''");
+      var twoEscapedQuotes = new RegExp(r"''");
       pattern = pattern.replaceAll(twoEscapedQuotes, "'");
     }
   }
 }
 
 /*
  * Represents a field in the pattern that formats some aspect of the
  * date. Consists primarily of a switch on the particular pattern characters
  * to determine what to do.
  */
@@ skipping 291 matching lines @@
     var basicString = toBePrinted.toString();
     if (basicString.length >= width) return basicString;
     var buffer = new StringBuffer();
     for (var i = 0; i < width - basicString.length; i++) {
       buffer.add('0');
      }
     buffer.add(basicString);
     return buffer.toString();
   }
 }