added a root isolate token test

lib/ui/fixtures/ui_test.dart

@@ -5,7 +5,7 @@
 import 'dart:async';
 import 'dart:typed_data';
 import 'dart:ui';
-import 'dart:isolate' show ReceivePort, SendPort;
+import 'dart:isolate';
 import 'dart:ffi';
 
 void main() {}
@@ -902,9 +902,25 @@ void hooksTests() {
     expectEquals(result, true);
   });
 
+  test('root isolate token', () async {
+    if (RootIsolateToken.instance == null) {
+      throw Exception('We should have a token on a root isolate.');
+    }
+    ReceivePort receivePort = ReceivePort();
+    Isolate.spawn(_backgroundRootIsolateTestMain, receivePort.sendPort);
+    bool didPass = await receivePort.first as bool;
+    if (!didPass) {
+      throw Exception('Background isolate found a root isolate id.');
+    }
+  });
+
   _finish();
 }
 
+void _backgroundRootIsolateTestMain(SendPort port) {
+  port.send(RootIsolateToken.instance == null);
+}
+
 typedef _Callback<T> = void Function(T result);
 typedef _Callbacker<T> = String? Function(_Callback<T?> callback);
 

lib/ui/platform_dispatcher.dart

@@ -81,7 +81,7 @@ class RootIsolateToken {
 
   /// The token for the root isolate that is executing this Dart code.  If this
   /// Dart code is not executing on a root isolate [instance] will be null.
-  late final RootIsolateToken? instance = () {
+  static late final RootIsolateToken? instance = () {
     final int rootIsolateId = __getRootIsolateId();
     return rootIsolateId == 0 ? null : RootIsolateToken._(rootIsolateId);
   }();