Revert "Remaining private libs"

sdk/lib/_internal/js_runtime/lib/regexp_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/regexp_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/regexp_helper.dart b/sdk/lib/_internal/js_runtime/lib/regexp_helper.dart
index b5fdcbd86da2e0db49fb532bfd79c8ac34afff74..efedcf164b163411704f13cbdad88a744b6253e3 100644
--- a/sdk/lib/_internal/js_runtime/lib/regexp_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/regexp_helper.dart
@@ -2,7 +2,7 @@
 // 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 "dart:_js_helper";
+part of "js_helper.dart";
 
 // Helper method used by internal libraries.
 regExpGetNative(JSSyntaxRegExp regexp) => regexp._nativeRegExp;