Remove deprecated 'str' getter on Match.

runtime/lib/string_patch.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.
 
 patch class String {
   /* patch */ factory String.fromCharCodes(Iterable<int> charCodes) {
     return _StringBase.createFromCharCodes(charCodes);
   }
 }
 
@@ skipping 608 matching lines @@
   }
 
   List<String> groups(List<int> groups) {
     List<String> result = new List<String>();
     for (int g in groups) {
       result.add(group(g));
     }
     return result;
   }
 
-  // TODO(12843): Remove when grace period is over.
-  String get str => input;
-
   final int start;
   final String input;
   final String pattern;
 }
 
 /**
  * An [Iterable] of the UTF-16 code units of a [String] in index order.
  */
 class _CodeUnits extends Object with ListMixin<int>,
                                      UnmodifiableListMixin<int> {
   /** The string that this is the code units of. */
   String _string;
 
   _CodeUnits(this._string);
 
   int get length => _string.length;
   int operator[](int i) => _string.codeUnitAt(i);
 }