Make int.gcd accept zero operands.

runtime/lib/bigint.dart

 // Copyright (c) 2014, 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.
 
 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 /*
  * Copyright (c) 2003-2005  Tom Wu
@@ skipping 1793 matching lines @@
     if (m == 1) return 0;
     m = m._toBigint();
     var t = this;
     if (t._neg || (t._absCompare(m) >= 0)) {
       t %= m;
       t = t._toBigint();
     }
     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);
   }
 }
 
 // Interface for modular reduction.
 class _Reduction {
   // Return the number of digits used by r_digits.
   int _convert(_Bigint x, Uint32List r_digits);
   int _mul(Uint32List x_digits, int x_used,
            Uint32List y_digits, int y_used, Uint32List r_digits);
@@ skipping 258 matching lines @@
 
   int _mul(Uint32List x_digits, int x_used,
            Uint32List y_digits, int y_used,
            Uint32List r_digits) {
     var r_used = _Bigint._mulDigits(x_digits, x_used,
                                     y_digits, y_used,
                                     r_digits);
     return _reduce(r_digits, r_used);
   }
 }