Add flutter mode to patched_sdk

pkg/front_end/lib/src/fasta/dill/dill_loader.dart

Index: pkg/front_end/lib/src/fasta/dill/dill_loader.dart
diff --git a/pkg/front_end/lib/src/fasta/dill/dill_loader.dart b/pkg/front_end/lib/src/fasta/dill/dill_loader.dart
index 7279ca74b2457c827053af5683c71bf94bf18057..e87c1a28548b4093e2b941266d2baf3e32143fb3 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_loader.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_loader.dart
@@ -8,6 +8,7 @@ import 'dart:async' show Future;
 
 import 'package:kernel/ast.dart' show Library, Program, Source;
 
+import '../errors.dart' show internalError;
 import '../loader.dart' show Loader;
 import '../target_implementation.dart' show TargetImplementation;
 import 'dill_library_builder.dart' show DillLibraryBuilder;
@@ -39,6 +40,9 @@ class DillLoader extends Loader<Library> {
   }
 
   Future<Null> buildOutline(DillLibraryBuilder builder) async {
+    if (builder.library == null) {
+      internalError("Builder.library for ${builder.uri} should not be null.");
+    }
     builder.library.classes.forEach(builder.addClass);
     builder.library.procedures.forEach(builder.addMember);
     builder.library.typedefs.forEach(builder.addTypedef);