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

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index f79dc5898e4a4257805531e7bea40532558d4d01..4eaf30692844fe9ee9b1ddd3cc03089e35914035 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -18808,7 +18808,9 @@ void Capability::PrintJSONImpl(JSONStream* stream, bool ref) const {
 }
 
 
-RawReceivePort* ReceivePort::New(Dart_Port id, Heap::Space space) {
+RawReceivePort* ReceivePort::New(Dart_Port id,
+                                 bool is_control_port,
+                                 Heap::Space space) {
   Isolate* isolate = Isolate::Current();
   const SendPort& send_port = SendPort::Handle(isolate, SendPort::New(id));
 
@@ -18821,7 +18823,11 @@ RawReceivePort* ReceivePort::New(Dart_Port id, Heap::Space space) {
     result ^= raw;
     result.raw_ptr()->send_port_ = send_port.raw();
   }
-  PortMap::SetLive(id);
+  if (is_control_port) {
+    PortMap::SetPortState(id, PortMap::kControlPort);
+  } else {
+    PortMap::SetPortState(id, PortMap::kLivePort);
+  }
   return result.raw();
 }