Add num.sign getter.

sdk/lib/core/num.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.
 
 part of dart.core;
 
 /**
  * An integer or floating-point number.
  *
  * It is a compile-time error for any type other than [int] or [double]
@@ skipping 152 matching lines @@
    *
    * The only non-finite numbers are NaN, positive infinitity and
    * negative infinity.
    */
   bool get isFinite;
 
   /** Returns the absolute value of this [num]. */
   num abs();
 
   /**
+   * Returns minus one, zero or plus one depending on the sign and
+   * numerical value of the number.
+   *
+   * Returns minus one if the number is less than zero,
+   * plus one if the number is greater than zero,
+   * and zero if the number is equal to zero.
+   *
+   * Returns NaN if the number is the double NaN value.
+   *
+   * Returns a number of the same type as this number.
+   * For doubles, `-0.0.sign == -0.0`.
+
+   * The result satisfies:
+   *
+   *     n == n.sign * n.abs()
+   *
+   * for all numbers `n` (except NaN, because NaN isn't `==` to itself).
+   */
+  num get sign;
+
+  /**
    * Returns the integer closest to `this`.
    *
    * Rounds away from zero when there is no closest integer:
    *  [:(3.5).round() == 4:] and [:(-3.5).round() == -4:].
    *
    * If `this` is not finite (`NaN` or infinity), throws an [UnsupportedError].
    */
   int round();
 
   /**
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     if (result != null) return result;
     result = double.parse(source, _returnNull);
     if (result != null) return result;
     if (onError == null) throw new FormatException(input);
     return onError(input);
   }
 
   /** Helper function for [parse]. */
   static _returnNull(_) => null;
 }