Introduce TypedefEntity

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

Index: pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
diff --git a/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart b/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
index 2b6c380ff945f161f0bc6cba2ab4756f522bd458..6802220b33442ced27b5f152a150d57d94d6e228 100644
--- a/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
+++ b/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
@@ -241,4 +241,11 @@ class KernelSorter implements Sorter {
             a, elementMap.getClassNode(a), b, elementMap.getClassNode(b));
       });
   }
+
+  @override
+  Iterable<TypedefEntity> sortTypedefs(Iterable<TypedefEntity> typedefs) {
+    // TODO(redemption): Support this.
+    assert(typedefs.isEmpty);
+    return typedefs;
+  }
 }