Introduce TypedefEntity

pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart

 // Copyright (c) 2017, 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 dart2js.kernel.backend_strategy;
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../backend_strategy.dart';
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
@@ skipping 223 matching lines @@
   @override
   Iterable<ClassEntity> sortClasses(Iterable<ClassEntity> classes) {
     return classes.toList()
       ..sort((ClassEntity a, ClassEntity b) {
         int r = _compareLibraries(a.library, b.library);
         if (r != 0) return r;
         return _compareNodes(
             a, elementMap.getClassNode(a), b, elementMap.getClassNode(b));
       });
   }
+
+  @override
+  Iterable<TypedefEntity> sortTypedefs(Iterable<TypedefEntity> typedefs) {
+    // TODO(redemption): Support this.
+    assert(typedefs.isEmpty);
+    return typedefs;
+  }
 }