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

sdk/lib/_internal/compiler/js_lib/js_string.dart

Index: sdk/lib/_internal/compiler/js_lib/js_string.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_string.dart b/sdk/lib/_internal/compiler/js_lib/js_string.dart
index e682626f8d7200b197e511320b2d27497687ed22..f7da941338759819188323fb4199c24bc9dbe48c 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_string.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_string.dart
@@ -97,6 +97,14 @@ class JSString extends Interceptor implements String, JSIndexable {
     }
   }
 
+  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.