First step towards asynchronous loading of sources

runtime/vm/isolate.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/isolate.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/json.h"
 #include "lib/mirrors.h"
@@ skipping 169 matching lines @@
     callback_name = isolate_->object_store()->unhandled_exception_handler();
   } else {
     callback_name = String::New("_unhandledExceptionCallback");
   }
   Library& lib =
       Library::Handle(isolate_, isolate_->object_store()->isolate_library());
   Function& func =
       Function::Handle(isolate_, lib.LookupLocalFunction(callback_name));
   if (func.IsNull()) {
     lib = isolate_->object_store()->root_library();
-    func = lib.LookupLocalFunction(callback_name);
+    // Note: bootstrap code in builtin library may attempt to resolve a
+    // callback function before the script is fully loaded, in which case
+    // the root library may not be registered yet.
+    if (!lib.IsNull()) {
+      func = lib.LookupLocalFunction(callback_name);
+    }
   }
   return func.raw();
 }
 
 
 bool IsolateMessageHandler::UnhandledExceptionCallbackHandler(
     const Object& message, const UnhandledException& error) {
   const Instance& cause = Instance::Handle(isolate_, error.exception());
   const Instance& stacktrace =
       Instance::Handle(isolate_, error.stacktrace());
@@ skipping 989 matching lines @@
   return func.raw();
 }
 
 
 void IsolateSpawnState::Cleanup() {
   SwitchIsolateScope switch_scope(isolate());
   Dart::ShutdownIsolate();
 }
 
 }  // namespace dart