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

sky/engine/core/script/dart_loader.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "sky/engine/config.h"
 #include "sky/engine/core/script/dart_loader.h"
 
 #include "base/callback.h"
 #include "base/trace_event/trace_event.h"
 #include "mojo/common/data_pipe_drainer.h"
@@ skipping 221 matching lines @@
 void DartLoader::WaitForDependencies(
     const HashSet<DartDependency*>& dependencies,
     const base::Closure& callback) {
   if (dependencies.isEmpty())
     return callback.Run();
   dependency_watchers_.add(
       adoptPtr(new DependencyWatcher(dependencies, callback)));
 }
 
 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));
     result.storedValue->value = job.get();
     jobs_.add(job.release());
   }
   if (dependency_catcher_)
     dependency_catcher_->AddDependency(result.storedValue->value);
 }
 
@@ skipping 51 matching lines @@
 
   WatcherSignaler watcher_signaler(*this, job);
 
   LOG(ERROR) << "Library Load failed: " << job->url().string().utf8().data();
   // TODO(eseidel): Call Dart_LibraryHandleError in the SourceJob case?
 
   jobs_.remove(job);
 }
 
 }  // namespace blink