Refactor isolate startup code in preparation for making isolate spawning more truly non-blocking.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index cede5f6bb91e25f995ec0afaf29fcae185a576ef..faabae860a5850d3f3068d5df0b197dba0de49bf 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -6925,7 +6925,9 @@ class ReceivePort : public Instance {
   static intptr_t InstanceSize() {
     return RoundedAllocationSize(sizeof(RawReceivePort));
   }
-  static RawReceivePort* New(Dart_Port id, Heap::Space space = Heap::kNew);
+  static RawReceivePort* New(Dart_Port id,
+                             bool is_control_port,
+                             Heap::Space space = Heap::kNew);
 
  private:
   FINAL_HEAP_OBJECT_IMPLEMENTATION(ReceivePort, Instance);