toRadixString returns lower case characters.

tests/corelib/src/IntegerToRadixStringTest.dart

Index: tests/corelib/src/IntegerToRadixStringTest.dart
diff --git a/tests/corelib/src/IntegerToRadixStringTest.dart b/tests/corelib/src/IntegerToRadixStringTest.dart
new file mode 100644
index 0000000000000000000000000000000000000000..35454bfde973d7ce67e3dd6e879076792c5e7f8c
--- /dev/null
+++ b/tests/corelib/src/IntegerToRadixStringTest.dart
@@ -0,0 +1,8 @@
+// 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.
+
+main() {
+  // Just make sure that we use lower-case characters.
+  Expect.equals("abcd", (0xabcd).toRadixString(16));
+}