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

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index b94d127c23fcce5ea39d0329831f305ab2dde2ae..7c338937bf41dff988e96e4ef8396206d44efdbb 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -803,12 +803,19 @@ class SwitchIsolateScope {
 
 class IsolateSpawnState {
  public:
-  explicit IsolateSpawnState(const Function& func);
-  explicit IsolateSpawnState(const char* script_url);
+  IsolateSpawnState(Dart_Port parent_port,
+                    const Function& func,
+                    const Instance& message);
+  IsolateSpawnState(Dart_Port parent_port,
+                    const char* script_url,
+                    const Instance& args,
+                    const Instance& message);
   ~IsolateSpawnState();
 
   Isolate* isolate() const { return isolate_; }
   void set_isolate(Isolate* value) { isolate_ = value; }
+
+  Dart_Port parent_port() const { return parent_port_; }
   char* script_url() const { return script_url_; }
   char* library_url() const { return library_url_; }
   char* class_name() const { return class_name_; }
@@ -817,15 +824,22 @@ class IsolateSpawnState {
   bool is_spawn_uri() const { return library_url_ == NULL; }
 
   RawObject* ResolveFunction();
+  RawInstance* BuildArgs();
+  RawInstance* BuildMessage();
   void Cleanup();
 
  private:
   Isolate* isolate_;
+  Dart_Port parent_port_;
   char* script_url_;
   char* library_url_;
   char* class_name_;
   char* function_name_;
   char* exception_callback_name_;
+  uint8_t* serialized_args_;
+  intptr_t serialized_args_len_;
+  uint8_t* serialized_message_;
+  intptr_t serialized_message_len_;
 };
 
 }  // namespace dart