Cache current thread in a reserved register and use it in LoadIsolate

runtime/vm/dart_entry.h

Index: runtime/vm/dart_entry.h
diff --git a/runtime/vm/dart_entry.h b/runtime/vm/dart_entry.h
index d8e86af1fdf1b4d81e97dba84600e1297a28ac48..f88a416f93134a4d6b1d4b0df6e87cabb7479934 100644
--- a/runtime/vm/dart_entry.h
+++ b/runtime/vm/dart_entry.h
@@ -102,7 +102,8 @@ class DartEntry : public AllStatic {
   // On success, returns a RawInstance.  On failure, a RawError.
   typedef RawObject* (*invokestub)(uword entry_point,
                                    const Array& arguments_descriptor,
-                                   const Array& arguments);
+                                   const Array& arguments,
+                                   Thread* thread);
 
   // Invokes the specified instance function or static function.
   // The first argument of an instance function is the receiver.