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

lib/src/runner/loader.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library test.runner.loader;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
 
@@ skipping 109 matching lines @@
       // the VM's or dart2js's.
       metadata = new Metadata();
     } on FormatException catch (error, stackTrace) {
       return new Stream.fromFuture(
           new Future.error(new LoadException(path, error), stackTrace));
     }
 
     var controller = new StreamController();
     Future.forEach(_platforms, (platform) {
       if (!metadata.testOn.evaluate(platform, os: currentOS)) {
-        return;
+        return null;
       }
 
       // Don't load a skipped suite.
       if (metadata.skip) {
         controller.add(new Suite([
           new LocalTest(path, metadata, () {})
         ], path: path, platform: platform.name, metadata: metadata));
-        return;
+        return null;
       }
 
       return new Future.sync(() {
         if (_pubServeUrl != null && !p.isWithin('test', path)) {
           throw new LoadException(path,
               'When using "pub serve", all test files must be in test/.');
         }
 
         if (platform == TestPlatform.vm) return _loadVmFile(path, metadata);
         assert(platform.isBrowser);
@@ skipping 90 matching lines @@
   Future close() {
     for (var isolate in _isolates) {
       isolate.kill();
     }
     _isolates.clear();
 
     if (_browserServerCompleter == null) return new Future.value();
     return _browserServer.then((browserServer) => browserServer.close());
   }
 }