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

tests/language/pure_function2_test.dart

Index: tests/language/pure_function2_test.dart
diff --git a/tests/language/pure_function2_test.dart b/tests/language/pure_function2_test.dart
index 1fdf5f076728d7c98475acf1e274bcf6195a8c30..c63d053d419822d2a42f4e0066de6e27f4e0b180 100644
--- a/tests/language/pure_function2_test.dart
+++ b/tests/language/pure_function2_test.dart
@@ -6,10 +6,9 @@ import "package:expect/expect.dart";
 
 // Regression test for issue 17483.
 
-confuse(x) {
-  if (new DateTime.now().millisecondsSinceEpoch == 42) return confuse(x);
-  return x;
-}
+
+@AssumeDynamic() @NoInline()
+confuse(x) => x;
 
 foo(trace) {
   trace.add("foo");