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

sdk/lib/_internal/compiler/js_lib/js_string.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 _interceptors;
 
 /**
  * The interceptor class for [String]. The compiler recognizes this
  * class as an interceptor, and changes references to [:this:] to
  * actually use the receiver of the method, which is generated as an extra
@@ skipping 79 matching lines @@
     if (pattern is String) {
       return JS('JSExtendableArray', r'#.split(#)', this, pattern);
     } else if (pattern is JSSyntaxRegExp && regExpCaptureCount(pattern) == 0) {
       var re = regExpGetNative(pattern);
       return JS('JSExtendableArray', r'#.split(#)', this, re);
     } else {
       return _defaultSplit(pattern);
     }
   }
 
+  String replaceRange(int start, int end, String replacement) {
+    checkString(replacement);
+    checkInt(start);
+    end = RangeError.checkValidRange(start, end, this.length);
+    checkInt(end);
+    return stringReplaceRangeUnchecked(this, start, end, replacement);
+  }
+
   List<String> _defaultSplit(Pattern pattern) {
     List<String> result = <String>[];
     // End of most recent match. That is, start of next part to add to result.
     int start = 0;
     // Length of most recent match.
     // Set >0, so no match on the empty string causes the result to be [""].
     int length = 1;
     for (var match in pattern.allMatches(this)) {
       int matchStart = match.start;
       int matchEnd = match.end;
@@ skipping 345 matching lines @@
   Type get runtimeType => String;
 
   int get length => JS('int', r'#.length', this);
 
   String operator [](int index) {
     if (index is !int) throw new ArgumentError(index);
     if (index >= length || index < 0) throw new RangeError.value(index);
     return JS('String', '#[#]', this, index);
   }
 }