Pass actual TestPlatform and OperatingSystem values to Suite.

lib/src/backend/metadata.dart

Index: lib/src/backend/metadata.dart
diff --git a/lib/src/backend/metadata.dart b/lib/src/backend/metadata.dart
index f8791ca4a571705096fb39d54b995f2c96421177..0dc2d2922b849d70ee0b7db52a3755f2271fdc63 100644
--- a/lib/src/backend/metadata.dart
+++ b/lib/src/backend/metadata.dart
@@ -161,6 +161,8 @@ class Metadata {
   /// Returns a copy of [this] with all platform-specific metadata from
   /// [onPlatform] resolved.
   Metadata forPlatform(TestPlatform platform, {OperatingSystem os}) {
+    if (onPlatform.isEmpty) return this;
+
     var metadata = this;
     onPlatform.forEach((platformSelector, platformMetadata) {
       if (!platformSelector.evaluate(platform, os: os)) return;