Fix bug in Duration.toString.

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 207 matching lines @@
 
   d = const Duration(hours: 1999, minutes: 17, seconds: 42);
   Expect.equals("1999:17:42.000000", d.toString());
 
   d = const Duration(days: -1, hours: -3, minutes: -17, seconds: -42,
                      milliseconds: -823, microseconds: -127);
   Expect.equals("-27:17:42.823127", d.toString());
 
   d = const Duration(hours: -1999, minutes: -17, seconds: -42);
   Expect.equals("-1999:17:42.000000", d.toString());
+
+  // Edge conditions for toString of microseconds.
+  // Regression test for http://dartbug.com/15678
+
+  d = const Duration(microseconds: 1);
+  Expect.equals("0:00:00.000001", d.toString());
+
+  d = const Duration(microseconds: 9);
+  Expect.equals("0:00:00.000009", d.toString());
+
+  d = const Duration(microseconds: 10);
+  Expect.equals("0:00:00.000010", d.toString());
+
+  d = const Duration(microseconds: 99);
+  Expect.equals("0:00:00.000099", d.toString());
+
+  d = const Duration(microseconds: 100);
+  Expect.equals("0:00:00.000100", d.toString());
+
+  d = const Duration(microseconds: 999);
+  Expect.equals("0:00:00.000999", d.toString());
+
+  d = const Duration(microseconds: 1000);
+  Expect.equals("0:00:00.001000", d.toString());
+
+  d = const Duration(microseconds: 9999);
+  Expect.equals("0:00:00.009999", d.toString());
+
+  d = const Duration(microseconds: 10000);
+  Expect.equals("0:00:00.010000", d.toString());
+
+  d = const Duration(microseconds: 99999);
+  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());
 }