At shutdown, remove isolate from list before tearing it down.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
===================================================================
--- runtime/vm/isolate.cc	(revision 37205)
+++ runtime/vm/isolate.cc	(working copy)
@@ -767,6 +767,10 @@
   ASSERT(top_resource() == NULL);
   ASSERT((heap_ == NULL) || heap_->Verify());
 
+  // Remove this isolate from the list *before* we start tearing it down, to
+  // avoid exposing it in a state of decay.
+  RemoveIsolateFromList(this);
+
   // Create an area where we do have a zone and a handle scope so that we can
   // call VM functions while tearing this isolate down.
   {
@@ -810,7 +814,6 @@
   // TODO(5411455): For now just make sure there are no current isolates
   // as we are shutting down the isolate.
   SetCurrent(NULL);
-  RemoveIsolateFromList(this);
   Profiler::ShutdownProfilingForIsolate(this);
 }