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

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 c10a42e4bf46410a3397923c5f3eb543a963ab08..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 "dart:_isolate_helper";
+part of _isolate_helper;
 
 /// Serialize [message].
 _serializeMessage(message) {