Fix all strong-mode errors and warnings.

lib/test.dart

 // Copyright (c) 2013, 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.
 
 import 'dart:async';
 
 import 'package:path/path.dart' as p;
 
 import 'src/backend/declarer.dart';
 import 'src/backend/test_platform.dart';
@@ skipping 47 matching lines @@
         os: currentOSGuess);
 
     var engine = new Engine();
     engine.suiteSink.add(suite);
     engine.suiteSink.close();
     ExpandedReporter.watch(engine,
         color: true, printPath: false, printPlatform: false);
 
     var success = await engine.run();
     // TODO(nweiz): Set the exit code on the VM when issue 6943 is fixed.
-    if (success) return;
+    if (success) return null;
     print('');
     new Future.error("Dummy exception to set exit code.");
   });
   return _globalDeclarer;
 }
 
 // TODO(nweiz): This and other top-level functions should throw exceptions if
 // they're called after the declarer has finished declaring.
 /// Creates a new test case with the given description (converted to a string)
 /// and body.
@@ skipping 161 matching lines @@
 /// prefer [tearDown], and only use [tearDOwnAll] if the callback is
 /// prohibitively slow.
 void tearDownAll(callback()) => _declarer.tearDownAll(callback);
 
 /// Registers an exception that was caught for the current test.
 void registerException(error, [StackTrace stackTrace]) {
   // This will usually forward directly to [Invoker.current.handleError], but
   // going through the zone API allows other zones to consistently see errors.
   Zone.current.handleUncaughtError(error, stackTrace);
 }