Make int.gcd accept zero operands.

sdk/lib/core/int.dart

Index: sdk/lib/core/int.dart
diff --git a/sdk/lib/core/int.dart b/sdk/lib/core/int.dart
index 72c5bcfe39215a2b7441d9c16e67817f9a14ca64..98f7da82da4bee10ca125d6a021837a91f525a3e 100644
--- a/sdk/lib/core/int.dart
+++ b/sdk/lib/core/int.dart
@@ -122,10 +122,17 @@ abstract class int extends num {
   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);