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

tests/language/null2_test.dart

Index: tests/language/null2_test.dart
diff --git a/tests/language/null2_test.dart b/tests/language/null2_test.dart
index 16d3a8308bfd7a64bdd72347190390aa0a52fcce..000eed68f12ef9002244b61b9eb38cad79dbadaf 100644
--- a/tests/language/null2_test.dart
+++ b/tests/language/null2_test.dart
@@ -5,16 +5,9 @@
 
 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());