Add tests for gcd, modInverse and modPow that also run on dart2js.

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 1544 matching lines @@
     if (inv) {
       if ((y_used == 1) && (y_digits[0] == 1)) return 1;
       if ((y_used == 0) || (y_digits[0].isEven && x_digits[0].isEven)) {
         throw new RangeError("Not coprime");
       }
     } else {
       if ((x_used == 0) || (y_used == 0)) throw new RangeError(0);
       if (((x_used == 1) && (x_digits[0] == 1)) ||
           ((y_used == 1) && (y_digits[0] == 1))) return 1;
       bool xy_cloned = false;
-      while (x.isEven && y.isEven) {
+      while (x_digits[0].isEven && y_digits[0].isEven) {

[regis, 2015/06/25 17:03:13]

Thanks
You could even write ((x_digits[0] & 1) == 0) && ((y_digits[0] & 1) == 0) as
below. 

[Lasse Reichstein Nielsen, 2015/06/26 08:40:40]

Done. 
Would it make sense to count the leading zero-bits and do one rsh, instead of
doing repeated rsh with 1? Or is this as rare a case?

         _rsh(x_digits, x_used, 1, x_digits);
         _rsh(y_digits, y_used, 1, y_digits);
         s++;
       }
       if (s >= _DIGIT_BITS) {
         var sd = s >> _LOG2_DIGIT_BITS;
         x_used -= sd;
         y_used -= sd;
         m_used -= sd;
       }
@@ skipping 494 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);
   }
 }