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

runtime/lib/bigint.dart

Index: runtime/lib/bigint.dart
diff --git a/runtime/lib/bigint.dart b/runtime/lib/bigint.dart
index adfb0db4cca73f924008dec248d1c07a09a464d5..a6c5f76e349b73e5384d4e3dbb229f3b03e38e8a 100644
--- a/runtime/lib/bigint.dart
+++ b/runtime/lib/bigint.dart
@@ -1562,7 +1562,7 @@ class _Bigint extends _IntegerImplementation implements int {
       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++;