Don't crash when main.dart is a 404.

sky/engine/core/script/dart_loader.cc

Index: sky/engine/core/script/dart_loader.cc
diff --git a/sky/engine/core/script/dart_loader.cc b/sky/engine/core/script/dart_loader.cc
index 773637a59789831f3d890e495baa385abf130375..f6efdb363dcf6bdc1cba3eba81363fe0d569fbe9 100644
--- a/sky/engine/core/script/dart_loader.cc
+++ b/sky/engine/core/script/dart_loader.cc
@@ -239,6 +239,11 @@ void DartLoader::WaitForDependencies(
 }
 
 void DartLoader::LoadLibrary(const KURL& url, mojo::URLResponsePtr response) {
+  if (response && response->status_code >= 400) {
+    LOG(ERROR) << url.string().utf8().data()
+        << " failed with " << response->status_code;
+  }
+
   const auto& result = pending_libraries_.add(url.string(), nullptr);
   if (result.isNewEntry) {
     OwnPtr<Job> job = adoptPtr(new ImportJob(this, url));