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

sdk/lib/_internal/js_runtime/lib/constant_map.dart

Index: sdk/lib/_internal/js_runtime/lib/constant_map.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/constant_map.dart b/sdk/lib/_internal/js_runtime/lib/constant_map.dart
index 215dcade5c45f668ae4b1d41f42c33f7c2c85dd2..618dc9604047bc255086c47c4a7e164be8688c6d 100644
--- a/sdk/lib/_internal/js_runtime/lib/constant_map.dart
+++ b/sdk/lib/_internal/js_runtime/lib/constant_map.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;
 
 class ConstantMapView<K, V> extends UnmodifiableMapView implements ConstantMap {
   ConstantMapView(Map base) : super(base);