Fix tests and analysis warnings and release 0.12.0-beta.9.

lib/src/runner/loader.dart

Index: lib/src/runner/loader.dart
diff --git a/lib/src/runner/loader.dart b/lib/src/runner/loader.dart
index 1b4aafa83bff29693e9556632b3b75b266e7589c..4b1f3d7447c1fe29874be04299603522abdc4156 100644
--- a/lib/src/runner/loader.dart
+++ b/lib/src/runner/loader.dart
@@ -127,7 +127,7 @@ class Loader {
     var controller = new StreamController();
     Future.forEach(_platforms, (platform) {
       if (!metadata.testOn.evaluate(platform, os: currentOS)) {
-        return;
+        return null;
       }
 
       // Don't load a skipped suite.
@@ -135,7 +135,7 @@ class Loader {
         controller.add(new Suite([
           new LocalTest(path, metadata, () {})
         ], path: path, platform: platform.name, metadata: metadata));
-        return;
+        return null;
       }
 
       return new Future.sync(() {