Create isolates in a separate thread.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index c97a102064ec27056e9e4b35d26502072aa32326..42f5b26b154150cf293c9d218cba592e18bf2906 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -807,9 +807,11 @@ class SwitchIsolateScope {
 class IsolateSpawnState {
  public:
   IsolateSpawnState(Dart_Port parent_port,
+                    void* init_data,
                     const Function& func,
                     const Instance& message);
   IsolateSpawnState(Dart_Port parent_port,
+                    void* init_data,
                     const char* script_url,
                     const Instance& args,
                     const Instance& message);
@@ -819,6 +821,7 @@ class IsolateSpawnState {
   void set_isolate(Isolate* value) { isolate_ = value; }
 
   Dart_Port parent_port() const { return parent_port_; }
+  void* init_data() const { return init_data_; }
   char* script_url() const { return script_url_; }
   char* library_url() const { return library_url_; }
   char* class_name() const { return class_name_; }
@@ -834,6 +837,7 @@ class IsolateSpawnState {
  private:
   Isolate* isolate_;
   Dart_Port parent_port_;
+  void* init_data_;
   char* script_url_;
   char* library_url_;
   char* class_name_;