Revert "Change some platform libraries to use `part of URI`."

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 7dfff6ecc387104889094aeb13ef9939451dbef5..e202f32c813f2607cbb6127b45c190acd15b47f4 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 "dart:_js_helper";
+part of _js_helper;
 
 stringIndexOfStringUnchecked(receiver, other, startIndex) {
   return JS('int', '#.indexOf(#, #)', receiver, other, startIndex);