Consistently use "OS" rather than "Os" in identifiers.

lib/unittest.dart

Index: lib/unittest.dart
diff --git a/lib/unittest.dart b/lib/unittest.dart
index bb01a81007a9fb6126e8cd398c8dd33947db518f..89dc3b5c9ee0c2570ac6763f839ba39491d21987 100644
--- a/lib/unittest.dart
+++ b/lib/unittest.dart
@@ -55,7 +55,7 @@ Declarer get _declarer {
       new Suite(_globalDeclarer.tests,
             path: p.prettyUri(Uri.base),
             platform: "VM")
-      .filter(TestPlatform.vm, os: currentOsGuess);
+      .filter(TestPlatform.vm, os: currentOSGuess);
     // TODO(nweiz): Set the exit code on the VM when issue 6943 is fixed.
     new NoIoCompactReporter([suite], color: true).run();
   });