Rework service extensions to be safe

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 3d06d6f091662382d079c13919042702373feb7c..1ae46a55dba047a01c0c88fa604de3aae51c5496 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -707,6 +707,9 @@ class Isolate : public BaseIsolate {
     compilation_allowed_ = allowed;
   }
 
+  RawObject* InvokePendingExtensionCalls();
+  void AppendExtensionCall(const Instance& closure);
+
 #if defined(DEBUG)
 #define REUSABLE_HANDLE_SCOPE_ACCESSORS(object)                                \
   void set_reusable_##object##_handle_scope_active(bool value) {               \
@@ -770,6 +773,12 @@ class Isolate : public BaseIsolate {
     user_tag_ = tag;
   }
 
+  RawGrowableObjectArray* GetAndClearPendingExtensionCalls();
+  RawGrowableObjectArray* pending_extension_calls() const {
+    return pending_extension_calls_;
+  }
+  void set_pending_extension_calls(const GrowableObjectArray& value);
+
   void ClearMutatorThread() {
     mutator_thread_ = NULL;
   }
@@ -861,6 +870,8 @@ class Isolate : public BaseIsolate {
   RawGrowableObjectArray* collected_closures_;
   RawGrowableObjectArray* deoptimized_code_array_;
 
+  RawGrowableObjectArray* pending_extension_calls_;
+
   Metric* metrics_list_head_;
 
   Counters counters_;