Make int.gcd accept zero operands.

tests/corelib/big_integer_arith_vm_test.dart

Index: tests/corelib/big_integer_arith_vm_test.dart
diff --git a/tests/corelib/big_integer_arith_vm_test.dart b/tests/corelib/big_integer_arith_vm_test.dart
index 4c7f12de9406021e8d6f4f844ef286ac52afe0f0..ad0f8f71ba2dda72023ba6da322314de4bcdcf26 100644
--- a/tests/corelib/big_integer_arith_vm_test.dart
+++ b/tests/corelib/big_integer_arith_vm_test.dart
@@ -310,9 +310,33 @@ testBigintGcd() {
   Expect.equals(21 <<40, x.gcd(m));
   x = 0;
   m = 1000000001;
-  Expect.throws(() => x.gcd(m), (e) => e is ArgumentError);
+  Expect.equals(m, x.gcd(m));
   x = 1000000001;
   m = 0;
+  Expect.equals(x, x.gcd(m));
+  x = 0;
+  m = -1000000001;
+  Expect.equals(-m, x.gcd(m));
+  x = -1000000001;
+  m = 0;
+  Expect.equals(-x, x.gcd(m));
+  x = 0;
+  m = 0;
+  Expect.throws(() => x.gcd(m), (e) => e is ArgumentError);
+  x = 0;
+  m = 123456789012345678901234567890;
+  Expect.equals(m, x.gcd(m));
+  x = 123456789012345678901234567890;
+  m = 0;
+  Expect.equals(x, x.gcd(m));
+  x = 0;
+  m = -123456789012345678901234567890;
+  Expect.equals(-m, x.gcd(m));
+  x = -123456789012345678901234567890;
+  m = 0;
+  Expect.equals(-x, x.gcd(m));
+  x = 0;
+  m = 0;
   Expect.throws(() => x.gcd(m), (e) => e is ArgumentError);
   x = 1234567890;
   m = 19;