Create isolates in a separate thread.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index dbe55885bca64729180e091184ca9963a83c2fd1..ac056f82ad9f3f3cb56cdcda41ef31af8389fc03 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1405,10 +1405,12 @@ static RawInstance* DeserializeObject(Isolate* isolate,
 
 
 IsolateSpawnState::IsolateSpawnState(Dart_Port parent_port,
+                                     void* init_data,
                                      const Function& func,
                                      const Instance& message)
     : isolate_(NULL),
       parent_port_(parent_port),
+      init_data_(init_data),
       script_url_(NULL),
       library_url_(NULL),
       class_name_(NULL),
@@ -1436,11 +1438,13 @@ IsolateSpawnState::IsolateSpawnState(Dart_Port parent_port,
 
 
 IsolateSpawnState::IsolateSpawnState(Dart_Port parent_port,
+                                     void* init_data,
                                      const char* script_url,
                                      const Instance& args,
                                      const Instance& message)
     : isolate_(NULL),
       parent_port_(parent_port),
+      init_data_(init_data),
       library_url_(NULL),
       class_name_(NULL),
       function_name_(NULL),