Pass actual TestPlatform and OperatingSystem values to Suite.

test/runner/loader_test.dart

Index: test/runner/loader_test.dart
diff --git a/test/runner/loader_test.dart b/test/runner/loader_test.dart
index 039055765c250013d7e186f21ccd990fbb461583..02e8af9dadad4885d38e089bd03fb7afa08ffb41 100644
--- a/test/runner/loader_test.dart
+++ b/test/runner/loader_test.dart
@@ -59,7 +59,7 @@ void main() {
 
     test("returns a suite with the file path and platform", () {
       expect(suite.path, equals(p.join(_sandbox, 'a_test.dart')));
-      expect(suite.platform, equals('VM'));
+      expect(suite.platform, equals(TestPlatform.vm));
     });
 
     test("returns tests with the correct names and platforms", () {