Change the badly named RegExp.pattern to return the original source.

sdk/lib/core/regexp.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.
 
 part of dart.core;
 
 /**
  * A result from searching within a string.
  *
  * A Match or an [Iterable] of Match objects is returned from [Pattern]
@@ skipping 131 matching lines @@
    * Returns whether the regular expression has a match in the string [input].
    */
   bool hasMatch(String input);
 
   /**
    * Returns the first substring match of this regular expression in [input].
    */
   String stringMatch(String input);
 
   /**
-   * The pattern of this regular expression.
+   * The source regular expression string used to create this `RegExp`.
    */
   String get pattern;
 
   /**
    * Whether this regular expression matches multiple lines.
    *
    * If the regexp does match multiple lines, the "^" and "$" characters
    * match the beginning and end of lines. If not, the character match the
    * beginning and end of the input.
    */
   bool get isMultiLine;
 
   /**
    * Whether this regular expression is case sensitive.
    *
    * If the regular expression is not case sensitive, it will match an input
    * letter with a pattern letter even if the two letters are different case
    * versions of the same letter.
    */
   bool get isCaseSensitive;
 }