Upgrade some tests to use annotations instead of 'clever' confuse() function

tests/language/null2_test.dart

 // Copyright (c) 2014, 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.
 // Second dart test program.
 
 import "package:expect/expect.dart";
 
-// Magic incantation to avoid the compiler recognizing the constant values
-// at compile time. If the result is computed at compile time, the dynamic code
-// will not be tested.
-confuse(x) {
-  try {
-    if (new DateTime.now().millisecondsSinceEpoch == 42) x = 42;
-    throw [x];
-  } on dynamic catch (e) { return e[0]; }
-  return 42;
-}
+@AssumeDynamic()
+@NoInline()
+confuse(x) => x;
 
 main() {
   Expect.equals("Null", null.runtimeType.toString());
   Expect.equals("Null", confuse(null).runtimeType.toString());
 }