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

sky/engine/core/script/dart_controller.cc

Index: sky/engine/core/script/dart_controller.cc
diff --git a/sky/engine/core/script/dart_controller.cc b/sky/engine/core/script/dart_controller.cc
index d024c441529794f94be1877265f11337e9cac589..04b03c117f329dbea5da92a91cdc32868e24a421 100644
--- a/sky/engine/core/script/dart_controller.cc
+++ b/sky/engine/core/script/dart_controller.cc
@@ -119,7 +119,9 @@ void DartController::DidLoadMainLibrary(KURL url) {
 
   Dart_Handle library = Dart_LookupLibrary(
       StringToDart(dart_state(), url.string()));
-  CHECK(!LogIfError(library));
+  // TODO(eseidel): We need to load a 404 page instead!
+  if (LogIfError(library))
+    return;
   DartInvokeAppField(library, ToDart("main"), 0, nullptr);
 }