Restructure/fix thread pointer visiting; added some asserts.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index ac444bde4efe132af08fd622d97c7a6333a0e2b0..9299ddd6a270c0efa67f234917f7b6a6b81fcef6 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -2604,6 +2604,9 @@ void Isolate::UnscheduleThread(Thread* thread,
       sticky_error_ = thread->sticky_error();
       thread->clear_sticky_error();
     }
+  } else {
+    ASSERT(thread->api_top_scope_ == NULL);
+    ASSERT(thread->zone_ == NULL);
   }
   if (!bypass_safepoint) {
     // Ensure that the thread reports itself as being at a safepoint.