Make int.gcd accept zero operands.

runtime/lib/bigint.dart

Index: runtime/lib/bigint.dart
diff --git a/runtime/lib/bigint.dart b/runtime/lib/bigint.dart
index 7e508bff07daa2aedfefcbffc7247e71bcf8e030..a278595175f561f9ca58520a3162aafafb66ebae 100644
--- a/runtime/lib/bigint.dart
+++ b/runtime/lib/bigint.dart
@@ -1811,11 +1811,14 @@ class _Bigint extends _IntegerImplementation implements int {
     return _binaryGcd(m, t, true);
   }
 
-  // Returns gcd of abs(this) and abs(other), with this != 0 and other !=0.
+  // Returns gcd of abs(this) and abs(other).
   int gcd(int other) {
     if (other is! int) {
       throw new ArgumentError.value(other, "other", "not an integer");
     }
+    if (other == 0) {
+      return this.abs();
+    }
     return _binaryGcd(this, other._toBigint(), false);
   }
 }