Enables clean VM shutdown.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 990d2339cb0632d122158610da12a7f96057e78e..54d8120285f03ef4273d8430ce49126c0c179309 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -754,6 +754,14 @@ class Isolate : public BaseIsolate {
     mutator_thread_->set_zone(zone);
   }
 
+  static void KillIsolate(Isolate* isolate);
+  static void KillAllIsolates();
+
+  static void DisableIsolateCreation();
+  static void EnableIsolateCreation();
+
+  static int IsolateCount();
+
  private:
   explicit Isolate(const Dart_IsolateFlags& api_flags);
 
@@ -914,12 +922,14 @@ 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;