Add a missing import to dart.dart.

This gives it access to [supportsIsolateKill].

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//975073002

lib/src/util/dart.dart

@@ -10,6 +10,7 @@ import 'dart:isolate';
 
 import 'package:path/path.dart' as p;
 
+import 'io.dart';
 import 'isolate_wrapper.dart';
 import 'remote_exception.dart';