Clean VM thread shutdown:

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index ecc59cba169b69428ff9542afd29764b5844fdbb..a6238fd8b388efedcfeda1b7b22d7810fbc4e8da 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -742,6 +742,12 @@ class Isolate : public BaseIsolate {
     mutator_thread_->set_zone(zone);
   }
 
+  static void KillIsolate(Isolate* isolate);
+  static void KillAllIsolates();
+
+  static void DisableIsolateCreation();
+  static void EnableIsolateCreation();
+
  private:
   friend class Dart;  // Init, InitOnce, Shutdown.
 
@@ -751,6 +757,7 @@ class Isolate : public BaseIsolate {
   static Isolate* Init(const char* name_prefix,
                        const Dart_IsolateFlags& api_flags,
                        bool is_vm_isolate = false);
+  void LowLevelShutdown();
   void Shutdown();
 
   void BuildName(const char* name_prefix);
@@ -917,12 +924,14 @@ class Isolate : public BaseIsolate {
   static void WakePauseEventHandler(Dart_Isolate isolate);
 
   // Manage list of existing isolates.
-  static void AddIsolateTolist(Isolate* isolate);
+  static bool AddIsolateToList(Isolate* isolate);
   static void RemoveIsolateFromList(Isolate* isolate);
   static void CheckForDuplicateThreadState(InterruptableThreadState* state);
 
-  static Monitor* isolates_list_monitor_;  // Protects isolates_list_head_
+  // This monitor protects isolates_list_head_, and creation_enabled_.
+  static Monitor* isolates_list_monitor_;
   static Isolate* isolates_list_head_;
+  static bool creation_enabled_;
 
 #define REUSABLE_FRIEND_DECLARATION(name)                                      \
   friend class Reusable##name##HandleScope;