Avoid bigints entering non-bigint gcd implementation.

runtime/lib/integers.dart

Index: runtime/lib/integers.dart
diff --git a/runtime/lib/integers.dart b/runtime/lib/integers.dart
index f465e3bd6bc27a8ccc00482e54e26281ae2ad725..e87dba700b759503669df13e4622ceee8f6541d1 100644
--- a/runtime/lib/integers.dart
+++ b/runtime/lib/integers.dart
@@ -394,8 +394,8 @@ abstract class _IntegerImplementation {
     if (x == 0) return y;
     if (y == 0) return x;
     if ((x == 1) || (y == 1)) return 1;
-    if (other is _Bigint) {
-      return _toBigint().gcd(other);
+    if (y is _Bigint) {
+      return x._toBigint().gcd(y);

[Lasse Reichstein Nielsen, 2016/12/06 09:05:23]

If other is -0x8000000000000000 (aka min-mint) then y == -other is a bigint, but
other is not. Similar for `this` and `x`. Might as well go to the bigint
implementation (and avoid doing the negation again).

[regis, 2016/12/06 17:08:15]

Agreed.

     }
     return _binaryGcd(x, y, false);
   }