Very simple version of Isolates.

runtime/lib/isolate.cc

Index: runtime/lib/isolate.cc
diff --git a/runtime/lib/isolate.cc b/runtime/lib/isolate.cc
index 9cfcddf0eee6bbc6327b54ad9428e2dd6daa22ce..29ab5b7f2d2fb6bf965a9a50460f2d5e0b2ead7a 100644
--- a/runtime/lib/isolate.cc
+++ b/runtime/lib/isolate.cc
@@ -215,7 +215,7 @@ static RawObject* Spawn(NativeArguments* arguments, IsolateSpawnState* state) {
 }
 
 
-DEFINE_NATIVE_ENTRY(isolate_spawnFunction, 2) {
+DEFINE_NATIVE_ENTRY(isolate_spawnFunction, 1) {
   GET_NON_NULL_NATIVE_ARGUMENT(Instance, closure, arguments->NativeArgAt(0));
   bool throw_exception = false;
   Function& func = Function::Handle();
@@ -235,36 +235,13 @@ DEFINE_NATIVE_ENTRY(isolate_spawnFunction, 2) {
     Exceptions::ThrowArgumentError(msg);
   }
 
-  GET_NATIVE_ARGUMENT(Instance, callback, arguments->NativeArgAt(1));
-  Function& callback_func = Function::Handle();
-  if (callback.IsClosure()) {
-    callback_func = Closure::function(callback);
-    const Class& cls = Class::Handle(callback_func.Owner());
-    if (!callback_func.IsClosureFunction() || !callback_func.is_static() ||
-        !cls.IsTopLevel()) {
-      throw_exception = true;
-    }
-  } else if (!callback.IsNull()) {
-    throw_exception = true;
-  }
-  if (throw_exception) {
-    const String& msg = String::Handle(String::New(
-        "spawnFunction expects to be passed either a unhandled exception "
-        "callback to a top-level static function, or null"));
-    Exceptions::ThrowArgumentError(msg);
-  }
-
 #if defined(DEBUG)
   Context& ctx = Context::Handle();
   ctx = Closure::context(closure);
   ASSERT(ctx.num_variables() == 0);
-  if (!callback.IsNull()) {
-    ctx = Closure::context(callback);
-    ASSERT(ctx.num_variables() == 0);
-  }
 #endif
 
-  return Spawn(arguments, new IsolateSpawnState(func, callback_func));
+  return Spawn(arguments, new IsolateSpawnState(func));
 }