Fix a couple of hints in the nnbd migrator

Change-Id: Iaa2442432fe300fd116224a2c2d75e9d76d4e1e2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/112260
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/nnbd_migration/lib/nnbd_migration.dart

@@ -48,7 +48,7 @@ class NullabilityFixDescription {
   /// nullable.
   factory NullabilityFixDescription.makeTypeNullable(String type) =>
       NullabilityFixDescription._(
-        appliedMessage: 'Changed type ${type} to be nullable',
+        appliedMessage: 'Changed type $type to be nullable',
       );
 
   const NullabilityFixDescription._({@required this.appliedMessage});

pkg/nnbd_migration/test/decorated_class_hierarchy_test.dart

@@ -3,9 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/ast/ast.dart';
-import 'package:analyzer/dart/element/type.dart';
 import 'package:nnbd_migration/src/decorated_class_hierarchy.dart';
-import 'package:nnbd_migration/src/decorated_type.dart';
 import 'package:nnbd_migration/src/nullability_node.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';