Fix issue 2888.

include/dart_api.h

Index: include/dart_api.h
===================================================================
--- include/dart_api.h	(revision 7254)
+++ include/dart_api.h	(working copy)
@@ -1916,13 +1916,28 @@
                                               Dart_Handle import_map);
 
 /**
+ * Set library tag handler for the current isolate. This handler is used to
+ * handle the various tags encountered while loading libraries or scripts in
+ * the isolate.
+ *
+ * \param handler Handler code to be used for handling the various tags
+ *   encountered while loading libraries or scripts in the isolate.
+ *
+ * \return If no error occurs, the handler is set for the isolate.
+ *   Otherwise an error handle is returned.
+ *
+ * TODO(turnidge): Document.
+ */
+DART_EXPORT Dart_Handle Dart_SetLibraryTagHandler(
+    Dart_LibraryTagHandler handler);
+
+/**
  * Loads the root script for the current isolate.
  *
  * TODO(turnidge): Document.
  */
 DART_EXPORT Dart_Handle Dart_LoadScript(Dart_Handle url,
                                         Dart_Handle source,
-                                        Dart_LibraryTagHandler handler,
                                         Dart_Handle import_map);
 
 /**