fix unit test

packages/file/test/memory_operations_test.dart

diff --git a/packages/file/test/memory_operations_test.dart b/packages/file/test/memory_operations_test.dart
index eb02d24..696a166 100644
--- a/packages/file/test/memory_operations_test.dart
+++ b/packages/file/test/memory_operations_test.dart
@@ -192,7 +192,7 @@
     List<FileSystemOp> operations = <FileSystemOp>[];
     MemoryFileSystem fs = MemoryFileSystem.test(
         opHandle: (String context, FileSystemOp operation) {
-      if (operation == FileSystemOp.create) {
+      if (operation == FileSystemOp.exists) {
         contexts.add(context);
         operations.add(operation);
       }