Change the thread interface in runtime/platform and use it starting all threads

runtime/lib/isolate.cc

Index: runtime/lib/isolate.cc
diff --git a/runtime/lib/isolate.cc b/runtime/lib/isolate.cc
index 75a9a34c3622a3671c7fcc314fd4d9e513482b8e..dad8a0654ebfea63c98838d00db12b63c1fd0799 100644
--- a/runtime/lib/isolate.cc
+++ b/runtime/lib/isolate.cc
@@ -298,7 +298,7 @@ DEFINE_NATIVE_ENTRY(IsolateNatives_start, 2) {
                                strdup(library_url),
                                strdup(class_name),
                                port_id));
-      new Thread(RunIsolate, data);
+      Thread::Start(RunIsolate, data);
     } else {
       // Error spawning the isolate, maybe due to initialization errors or
       // errors while loading the application into spawned isolate, shut