Read SDK and patches from a JSON file.

pkg/front_end/lib/src/fasta/dill/dill_target.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_target;
 
 import 'dart:async' show Future;
 
 import 'package:kernel/ast.dart' show Class;
 
@@ skipping 32 matching lines @@
   }
 
   @override
   Future<Null> buildOutlines() async {
     if (loader.libraries.isNotEmpty) {
       await loader.buildOutlines();
     }
     isLoaded = true;
   }
 
-  DillLibraryBuilder createLibraryBuilder(Uri uri, Uri fileUri) {
+  DillLibraryBuilder createLibraryBuilder(Uri uri, Uri fileUri, bool isPatch) {
+    assert(!isPatch);
     return new DillLibraryBuilder(uri, loader);
   }
 
   void addDirectSupertype(ClassBuilder cls, Set<ClassBuilder> set) {}
 
   List<ClassBuilder> collectAllClasses() {
     return null;
   }
 
   void breakCycle(ClassBuilder cls) {}
 
   Class get objectClass => loader.coreLibrary["Object"].target;
 }