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

sdk/lib/_internal/js_runtime/lib/isolate_serialization.dart

Index: sdk/lib/_internal/js_runtime/lib/isolate_serialization.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/isolate_serialization.dart b/sdk/lib/_internal/js_runtime/lib/isolate_serialization.dart
index dfa854a863eb5bd17afb0c352580f9cd903f0a77..619ef6ad2820b09294f6bb87976433a7df6ab863 100644
--- a/sdk/lib/_internal/js_runtime/lib/isolate_serialization.dart
+++ b/sdk/lib/_internal/js_runtime/lib/isolate_serialization.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 "isolate_helper.dart";
+part of _isolate_helper;
 
 /// Serialize [message].
 _serializeMessage(message) {