DateTime and Duration tweaks for better dart2js codegen

sdk/lib/core/duration.dart

 // Copyright (c) 2011, 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.
 
 part of dart.core;
 
 /**
  * A span of time, such as 27 days, 4 hours, 12 minutes, and 3 seconds.
  *
  * A `Duration` represents a difference from one point in time to another. The
@@ skipping 277 matching lines @@
    * positive.
    */
   Duration abs() => new Duration._microseconds(_duration.abs());
 
   /**
    * Returns a new `Duration` representing this `Duration` negated.
    *
    * The returned `Duration` has the same length as this one, but will have the
    * opposite sign of this one.
    */
-  Duration operator -() => new Duration._microseconds(-_duration);
+  // Using subtraction helps dart2js avoid negative zeros.
+  Duration operator -() => new Duration._microseconds(0 - _duration);
 }