Per isolate package root.

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
diff --git a/runtime/include/dart_api.h b/runtime/include/dart_api.h
index d56241d32cccf1a1a4c682a05ca4e4a13adcd10d..dbceebefbdf282d700a0a2c4e9ecd5a5e6c38931 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -720,6 +720,9 @@ DART_EXPORT const char* Dart_VersionString();
  *   eventually run.  This is provided for advisory purposes only to
  *   improve debugging messages.  The main function is not invoked by
  *   this function.
+ * \param package_root The package root path for this isolate to resolve
+ *   package imports against. If this parameter is NULL, the package root path
+ *   of the parent isolate should be used.
  * \param callback_data The callback data which was passed to the
  *   parent isolate when it was created by calling Dart_CreateIsolate().
  * \param error A structure into which the embedder can place a
@@ -730,6 +733,7 @@ DART_EXPORT const char* Dart_VersionString();
  */
 typedef Dart_Isolate (*Dart_IsolateCreateCallback)(const char* script_uri,
                                                    const char* main,
+                                                   const char* package_root,
                                                    void* callback_data,
                                                    char** error);