Revert "Use relative URIs for core library part-of statements."

sdk/lib/_internal/js_runtime/lib/string_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/string_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/string_helper.dart b/sdk/lib/_internal/js_runtime/lib/string_helper.dart
index 81099bdd532b4dc40a2cc448071dc95bd23271e8..cf8b17ad2459f3a6b8f8249c25bf6776c57503b0 100644
--- a/sdk/lib/_internal/js_runtime/lib/string_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/string_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 "js_helper.dart";
+part of _js_helper;
 
 stringIndexOfStringUnchecked(receiver, other, startIndex) {
   return JS('int', '#.indexOf(#, #)', receiver, other, startIndex);