Fix isolate tests on Dart SDK 2.14 and older.

Author: greenrobot-team

objectbox/test/isolates_test.dart

@@ -49,20 +49,24 @@ void main() {
   /// Work with a single store across multiple isolates using
   /// the legacy way of passing a pointer reference to the isolate.
   test('single store using reference', () async {
-    final storeCreator = (dynamic msg) =>
-        Store.fromReference(getObjectBoxModel(), msg as ByteData);
-    await testUsingStoreFromIsolate(storeCreator, (env) => env.store.reference);
+    await testUsingStoreFromIsolate(
+        storeCreatorFromRef, (env) => env.store.reference);
   });
 
   /// Work with a single store across multiple isolates using
   /// the directory path to attach to an existing store.
   test('single store using attach', () async {
-    final storeCreator =
-        (dynamic msg) => Store.attach(getObjectBoxModel(), msg as String);
-    await testUsingStoreFromIsolate(storeCreator, (env) => env.dir.path);
+    await testUsingStoreFromIsolate(storeCreatorAttach, (env) => env.dir.path);
   });
 }
 
+// Note: can't use closures, are only supported from Dart SDK 2.15.
+Store storeCreatorFromRef(dynamic msg) =>
+    Store.fromReference(getObjectBoxModel(), msg as ByteData);
+
+Store storeCreatorAttach(dynamic msg) =>
+    Store.attach(getObjectBoxModel(), msg as String);
+
 class IsolateInitMessage {
   SendPort sendPort;
   Store Function(dynamic) storeCreator;