Make int.gcd accept zero operands.

runtime/lib/integers.dart

Index: runtime/lib/integers.dart
diff --git a/runtime/lib/integers.dart b/runtime/lib/integers.dart
index f31a453f437920a1f88a633e8647bbc31fe5e295..7685ca60ed18fdb475c92bc04f475cfcc28fb8d0 100644
--- a/runtime/lib/integers.dart
+++ b/runtime/lib/integers.dart
@@ -391,14 +391,14 @@ class _IntegerImplementation extends _Num {
     if (other is! int) {
       throw new ArgumentError.value(other, "other", "not an integer");
     }
-    if (this == 0) {
-      throw new ArgumentError.value(this, "first operand", "must not be zero");
-    }
-    if (other == 0) {
-      throw new ArgumentError.value(this, "second operand", "must not be zero");
+    if ((this == 0) && (other == 0)) {
+      throw new ArgumentError.value(0, null,
+                                    "at least one operand must not be zero");
     }
     int x = this.abs();
     int y = other.abs();
+    if (x == 0) return y;
+    if (y == 0) return x;
     if ((x == 1) || (y == 1)) return 1;
     if (other is _Bigint) {
       return _toBigint().gcd(other);