Add num.sign getter.

runtime/lib/double.dart

 // 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.
 
 class _Double implements double {
   factory _Double.fromInteger(int value)
       native "Double_doubleFromInteger";
 
   Type get runtimeType => double;
 
@@ skipping 86 matching lines @@
   bool get isInfinite native "Double_getIsInfinite";
   bool get isNaN native "Double_getIsNaN";
   bool get isFinite => !isInfinite && !isNaN;  // Can be optimized.
 
   double abs() {
     // Handle negative 0.0.
     if (this == 0.0) return 0.0;
     return this < 0.0 ? -this : this;
   }
 
+  double get sign {
+    if (this > 0.0) return 1.0;
+    if (this < 0.0) return -1.0;
+    return this;  // +/-0.0 or NaN.
+  }
+
   int round() => roundToDouble().toInt();
   int floor() => floorToDouble().toInt();
   int ceil () => ceilToDouble().toInt();
   int truncate() => truncateToDouble().toInt();
 
   double roundToDouble() native "Double_round";
   double floorToDouble() native "Double_floor";
   double ceilToDouble() native "Double_ceil";
   double truncateToDouble() native "Double_truncate";
 
@@ skipping 113 matching lines @@
         return EQUAL;
       }
     } else if (isNaN) {
       return other.isNaN ? EQUAL : GREATER;
     } else {
       // Other is NaN.
       return LESS;
     }
   }
 }