DateTime and Duration tweaks for better dart2js codegen

sdk/lib/core/duration.dart

Index: sdk/lib/core/duration.dart
diff --git a/sdk/lib/core/duration.dart b/sdk/lib/core/duration.dart
index f2dd01b331eda4fd350d41001d5e2686db7d2760..bc1364fba3e3daf3bd71a2a3b76ddc17b392343a 100644
--- a/sdk/lib/core/duration.dart
+++ b/sdk/lib/core/duration.dart
@@ -295,5 +295,6 @@ class Duration implements Comparable<Duration> {
    * 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);
 }