Add a callback to inform embedders when an isolate is being shut down.

vm/isolate.cc

Index: vm/isolate.cc
===================================================================
--- vm/isolate.cc	(revision 7933)
+++ vm/isolate.cc	(working copy)
@@ -393,28 +393,9 @@
 
 Dart_IsolateCreateCallback Isolate::create_callback_ = NULL;
 Dart_IsolateInterruptCallback Isolate::interrupt_callback_ = NULL;
+Dart_IsolateShutdownCallback Isolate::shutdown_callback_ = NULL;
 
 
-void Isolate::SetCreateCallback(Dart_IsolateCreateCallback cb) {
-  create_callback_ = cb;
-}
-
-
-Dart_IsolateCreateCallback Isolate::CreateCallback() {
-  return create_callback_;
-}
-
-
-void Isolate::SetInterruptCallback(Dart_IsolateInterruptCallback cb) {
-  interrupt_callback_ = cb;
-}
-
-
-Dart_IsolateInterruptCallback Isolate::InterruptCallback() {
-  return interrupt_callback_;
-}
-
-
 void Isolate::VisitObjectPointers(ObjectPointerVisitor* visitor,
                                   bool visit_prologue_weak_handles,
                                   bool validate_frames) {