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

bin/run_vm_tests.cc

Index: bin/run_vm_tests.cc
===================================================================
--- bin/run_vm_tests.cc	(revision 7933)
+++ bin/run_vm_tests.cc	(working copy)
@@ -100,7 +100,7 @@
   bool set_vm_flags_success = Flags::ProcessCommandLineFlags(dart_argc,
                                                              dart_argv);
   ASSERT(set_vm_flags_success);
-  bool init_success = Dart::InitOnce(NULL, NULL);
+  bool init_success = Dart::InitOnce(NULL, NULL, NULL);
   ASSERT(init_success);
   // Apply the filter to all registered tests.
   TestCaseBase::RunAll();