Fix test to not reach out of current dir

tests/corelib/string_operations_with_null_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 "package:expect/expect.dart";
-import "../language/compiler_annotations.dart";
 
-@DontInline()
+@NoInline()
+@AssumeDynamic()
 returnStringOrNull() {
-  () => 42;
   return new DateTime.now().millisecondsSinceEpoch == 0 ? 'foo' : null;
 }
 
 main() {
   Expect.throws(() => 'foo' + returnStringOrNull(),
                 (e) => e is ArgumentError);
   Expect.throws(() => 'foo'.split(returnStringOrNull()),
                 (e) => e is ArgumentError || e is NoSuchMethodError);
 }