Retain configured file reporters when resolving presets. (#1217)

diff --git a/pkgs/test_core/lib/src/runner/configuration.dart b/pkgs/test_core/lib/src/runner/configuration.dart
index e442f91..5efad44 100644
--- a/pkgs/test_core/lib/src/runner/configuration.dart
+++ b/pkgs/test_core/lib/src/runner/configuration.dart
@@ -536,6 +536,7 @@
       String configurationPath,
       String dart2jsPath,
       String reporter,
+      Map<String, String> fileReporters,
       int pubServePort,
       int concurrency,
       int shardIndex,
@@ -581,6 +582,7 @@
         configurationPath: configurationPath ?? _configurationPath,
         dart2jsPath: dart2jsPath ?? _dart2jsPath,
         reporter: reporter ?? _reporter,
+        fileReporters: fileReporters ?? this.fileReporters,
         pubServePort: pubServePort ?? pubServeUrl?.port,
         concurrency: concurrency ?? _concurrency,
         shardIndex: shardIndex ?? this.shardIndex,