Fix typo.

tests/language/is_nan_test.dart

Index: tests/language/is_nan_test.dart
===================================================================
--- tests/language/is_nan_test.dart	(revision 0)
+++ tests/language/is_nan_test.dart	(revision 0)
@@ -0,0 +1,15 @@
+// Copyright (c) 2012, 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.
+
+class A {
+  bool isNaN() => false;
+}
+
+main() {
+  Expect.isTrue(foo(double.NAN));
+  Expect.isFalse(foo(new A()));
+  Expect.throws(() => foo('bar'), (e) => e is NoSuchMethodException);
+}
+
+foo(a) => a.isNaN();