[gardening] Ensure path is converted to uri in reload tests.

Follow-up to 91c628d

Change-Id: I2504d0b69c060b5d77002b3b0c58ec0ff6e0a4af
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/188640
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

runtime/tests/vm/dart_2/isolates/reload_utils.dart

@@ -25,7 +25,7 @@ Future<List<String>> generateDills(String tempDir, String testDartFile) async {
     await File(testFile).writeAsString(version);
     final dillFile = path.join(tempDir, 'test.dart.${i++}.dill');
     await compile(testFile, dillFile);
-    dills.add(dillFile);
+    dills.add(Uri.file(dillFile).toString());
   }
   return dills;
 }