Add String.replaceRange and use it in replaceFirst{,Mapped}.

sdk/lib/_internal/compiler/js_lib/string_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;
 
 class StringMatch implements Match {
   const StringMatch(int this.start,
                     String this.input,
                     String this.pattern);
@@ skipping 64 matching lines @@
   // behavior.
   to = JS('String', r'#.replace(/\$/g, "$$$$")', to);
   return JS('String', r'#.replace(#, #)', receiver, replacer, to);
 }
 
 stringReplaceFirstRE(receiver, regexp, to, startIndex) {
   var match = regexp._execGlobal(receiver, startIndex);
   if (match == null) return receiver;
   var start = match.start;
   var end = match.end;
-  return "${receiver.substring(0,start)}$to${receiver.substring(end)}";
+  return stringReplaceRangeUnchecked(receiver, start, end, to);
 }
 
 const String ESCAPE_REGEXP = r'[[\]{}()*+?.\\^$|]';
 
 stringReplaceAllUnchecked(receiver, from, to) {
   checkString(to);
   if (from is String) {
     if (from == "") {
       if (receiver == "") {
         return to;
@@ skipping 94 matching lines @@
   }
   buffer.write(onNonMatch(receiver.substring(startIndex)));
   return buffer.toString();
 }
 
 
 stringReplaceFirstUnchecked(receiver, from, to, int startIndex) {
   if (from is String) {
     int index = receiver.indexOf(from, startIndex);
     if (index < 0) return receiver;
-    return '${receiver.substring(0, index)}$to'
-           '${receiver.substring(index + from.length)}';
+    int end = index + from.length;
+    return stringReplaceRangeUnchecked(receiver, index, end, to);
   }
   if (from is JSSyntaxRegExp) {
     return startIndex == 0 ?
         stringReplaceJS(receiver, regExpGetNative(from), to) :
         stringReplaceFirstRE(receiver, from, to, startIndex);
   }
   checkNull(from);
   Iterator<Match> matches = from.allMatches(receiver, startIndex).iterator;
   if (!matches.moveNext()) return receiver;
   Match match = matches.current;
   return '${receiver.substring(0, match.start)}$to'
          '${receiver.substring(match.end)}';
 }
 
 stringReplaceFirstMappedUnchecked(receiver, from, replace,
                                   int startIndex) {
   Iterator<Match> matches = from.allMatches(receiver, startIndex).iterator;
   if (!matches.moveNext()) return receiver;
   Match match = matches.current;
   String replacement = "${replace(match)}";
   return '${receiver.substring(0, match.start)}$replacement'
          '${receiver.substring(match.end)}';
 }
 
 stringJoinUnchecked(array, separator) {
   return JS('String', r'#.join(#)', array, separator);
 }
+
+String stringReplaceRangeUnchecked(String receiver,
+                                   int start, int end, String replacement) {
+  var prefix = JS('String', '#.substring(0, #)', receiver, start);
+  var suffix = JS('String', '#.substring(#)', receiver, end);
+  return "$prefix$replacement$suffix";
+}