Make int.gcd accept zero operands.

tests/corelib/big_integer_arith_vm_test.dart

 // Copyright (c) 2011, 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.
 
 // Testing Bigints with and without intrinsics.
 // VMOptions=
 // VMOptions=--no_intrinsify
 // VMOptions=--optimization_counter_threshold=10
 
 library big_integer_test;
@@ skipping 292 matching lines @@
   m = 609;
   Expect.equals(21, x.gcd(m));
   x = 693 << 40;
   m = 609 << 40;
   Expect.equals(21 << 40, x.gcd(m));
   x = 609 << 40;;
   m = 693 << 40;;
   Expect.equals(21 <<40, 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 = -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 = 1000000001;
+  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;
   Expect.equals(1, x.gcd(m));
   x = 1234567890;
   m = 1000000001;
   Expect.equals(1, x.gcd(m));
   x = 19;
   m = 1000000001;
@@ skipping 88 matching lines @@
     testBigintNegate();  /// negate: ok
     testShiftAmount();  /// shift: ok
     Expect.equals(12345678901234567890, (12345678901234567890).abs());
     Expect.equals(12345678901234567890, (-12345678901234567890).abs());
     var a = 10000000000000000000;
     var b = 10000000000000000001;
     Expect.equals(false, a.hashCode == b.hashCode);
     Expect.equals(true, a.hashCode == (b - 1).hashCode);
   }
 }