Support loading dart-ext from within a package when using a package map

runtime/bin/loader.h

Index: runtime/bin/loader.h
diff --git a/runtime/bin/loader.h b/runtime/bin/loader.h
index 1c8f4ad99497bfc9feb152a02ba58e9f72d0c050..97e87a409a9c4298f81a604b8a81f2210410611b 100644
--- a/runtime/bin/loader.h
+++ b/runtime/bin/loader.h
@@ -84,6 +84,10 @@ class Loader {
             const char* working_directory,
             const char* root_script_uri);
 
+  // Send a request for a dart-ext: import to the service isolate.
+  void SendImportExtensionRequest(Dart_Handle url,
+                                  Dart_Handle library_url);
+
   // Send a request from the tag handler to the service isolate.
   void SendRequest(Dart_LibraryTag tag,
                    Dart_Handle url,