Make int.gcd accept zero operands.

sdk/lib/core/int.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 arbitrarily large integer.
  *
  * **Note:** When compiling to JavaScript, integers are
@@ skipping 104 matching lines @@
    * Returns the modular multiplicative inverse of this integer
    * modulo [modulus].
    *
    * The [modulus] must be positive.
    *
    * It is an error if no modular inverse exists.
    */
   int modInverse(int modulus);
 
   /**
-   * Returns the greatest common divisor of the absolute value of
-   * this integer and the absolute value of [other].
+   * Returns the greatest common divisor of this integer and [other].
    *
-   * Both this and [other] must be non-zero.
+   * If either number is non-zero, the result is the numerically greatest
+   * integer dividing both `this` and `other`.
+   *
+   * The greatest common divisor is independent of the order,
+   * so `x.gcd(y)` is  always the same as `y.gcd(x)`.
+   *
+   * For any integer `x`, `x.gcd(x)` is `x.abs()`.
+   *
+   * If both `this` and `other` is zero, the result is also zero.
    */
   int gcd(int other);
 
   /** Returns true if and only if this integer is even. */
   bool get isEven;
 
   /** Returns true if and only if this integer is odd. */
   bool get isOdd;
 
   /**
@@ skipping 165 matching lines @@
    *     var value = int.parse(text, onError: (source) => null);
    *     if (value == null) ... handle the problem
    *
    * The [onError] function is only invoked if [source] is a [String]. It is
    * not invoked if the [source] is, for example, `null`.
    */
   external static int parse(String source,
                             { int radix,
                               int onError(String source) });
 }