Fix implementations of Match too, to use "input", not "str".

sdk/lib/_internal/lib/regexp_helper.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 _js_helper;
 
 // Helper method used by internal libraries.
 regExpGetNative(JSSyntaxRegExp regexp) => regexp._nativeRegExp;
 
 /**
@@ skipping 132 matching lines @@
   final Pattern pattern;
   // Contains a JS RegExp match object.
   // It is an Array of String values with extra "index" and "input" properties.
   final List<String> _match;
 
   _MatchImplementation(this.pattern, this._match) {
     assert(JS("var", "#.input", _match) is String);
     assert(JS("var", "#.index", _match) is int);
   }
 
-  String get str => JS("String", "#.input", _match);
+  // TODO(12843): Remove when grace period is over.
+  String get str => input;
+  String get input => JS("String", "#.input", _match);
   int get start => JS("int", "#.index", _match);
   int get end => start + _match[0].length;
 
   String group(int index) => _match[index];
   String operator [](int index) => group(index);
   int get groupCount => _match.length - 1;
 
   List<String> groups(List<int> groups) {
     List<String> out = [];
     for (int i in groups) {
@@ skipping 36 matching lines @@
       return false;
     }
     return true;
   }
 }
 
 /** Find the first match of [regExp] in [string] at or after [start]. */
 Match firstMatchAfter(JSSyntaxRegExp regExp, String string, int start) {
   return regExp._execGlobal(string, start);
 }