Add flutter mode to patched_sdk

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

 // Copyright (c) 2016, 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.
 
 library fasta.dill_loader;
 
 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;
 
 class DillLoader extends Loader<Library> {
   /// Source targets are compiled against these binary libraries.
   final libraries = <Library>[];
 
   /// Sources for all appended programs.
   final Map<String, Source> uriToSource = <String, Source>{};
@@ skipping 11 matching lines @@
         DillLibraryBuilder builder = read(library.importUri, -1);
         builder.library = library;
         builders.add(builder);
       }
     }
     uriToSource.addAll(program.uriToSource);
     return builders;
   }
 
   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);
     builder.library.fields.forEach(builder.addMember);
   }
 
   Future<Null> buildBody(DillLibraryBuilder builder) {
     return buildOutline(builder);
   }
 }