Adding Duration documentation, abs(), and isNegative()

tests/corelib/duration_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";
 
 main() {
   Duration d;
   d = new Duration(days: 1);
   Expect.equals(86400000000, d.inMicroseconds);
@@ skipping 249 matching lines @@
   Expect.equals("0:00:00.099999", d.toString());
 
   d = const Duration(microseconds: 100000);
   Expect.equals("0:00:00.100000", d.toString());
 
   d = const Duration(microseconds: 999999);
   Expect.equals("0:00:00.999999", d.toString());
 
   d = const Duration(microseconds: 1000000);
   Expect.equals("0:00:01.000000", d.toString());
+
+  d1 = const Duration(hours: 1);
+  d2 = const Duration(hours: -1);
+  Expect.isFalse(d1.isNegative());
+  Expect.isTrue(d2.isNegative());
+  Expect.equals(d1, d1.abs());
+  Expect.equals(d1, d2.abs());
 }