Remove workaround in gcd for even operands and provide permanent fix.

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 276 matching lines @@
   }
 
   // Return this << n.
   _Bigint _lShift(int n) {
     final ds = n ~/ _DIGIT_BITS;
     final bs = n % _DIGIT_BITS;
     if (bs == 0) {
       return _dlShift(ds);
     }
     var r_used = _used + ds + 1;
-    var r_digits = new Uint32List(r_used + 2 + (r_used & 1));  // +2 for 64-bit.
+    var r_digits = new Uint32List(r_used + 2 - (r_used & 1));  // for 64-bit.
     _lsh(_digits, _used, n, r_digits);
     return new _Bigint(_neg, r_used, r_digits);
   }
 
   // r_digits[0..r_used-1] = x_digits[0..x_used-1] << n.
   // Return r_used.
   static int _lShiftDigits(Uint32List x_digits, int x_used, int n,
                            Uint32List r_digits) {
     final ds = n ~/ _DIGIT_BITS;
     final bs = n % _DIGIT_BITS;
     if (bs == 0) {
       return _dlShiftDigits(x_digits, x_used, ds, r_digits);
     }
     var r_used = x_used + ds + 1;
-    assert(r_digits.length >= r_used + 2 + (r_used & 1));  // +2 for 64-bit.
+    assert(r_digits.length >= r_used + 2 - (r_used & 1));  // for 64-bit.
     _lsh(x_digits, x_used, n, r_digits);
     var i = ds;
     while (--i >= 0) {
       r_digits[i] = 0;
     }
     if (r_digits[r_used - 1] == 0) {
       r_used--;  // Clamp result.
     } else if (r_used.isOdd) {
       r_digits[r_used] = 0;
     }
@@ skipping 1255 matching lines @@
       if ((y_digits[0] & 1) == 1) {
         var t_digits = x_digits;
         var t_used = x_used;
         x_digits = y_digits;
         x_used = y_used;
         y_digits = t_digits;
         y_used = t_used;
       }
     }
     var u_digits = _cloneDigits(x_digits, 0, x_used, m_len);
-    var v_digits = _cloneDigits(y_digits, 0, y_used, m_len);
+    var v_digits = _cloneDigits(y_digits, 0, y_used, m_len + 2);  // +2 for lsh.
     final bool ac = (x_digits[0] & 1) == 0;
 
     // Variables a, b, c, and d require one more digit.
     final abcd_used = m_used + 1;
     final abcd_len = abcd_used + (abcd_used & 1) + 2;  // +2 to satisfy _absAdd.
     var a_digits, b_digits, c_digits, d_digits;
     bool a_neg, b_neg, c_neg, d_neg;
     if (ac) {
       a_digits = new Uint32List(abcd_len);
       a_neg = false;
@@ skipping 142 matching lines @@
         } else {
           _absAdd(d_digits, abcd_used, b_digits, abcd_used, d_digits);
         }
       }
       // Exit loop if u == 0.
       var i = m_used;
       while ((i > 0) && (u_digits[i - 1] == 0)) --i;
       if (i == 0) break;
     }
     if (!inv) {
-      // TODO(regis): Make this work correctly:
-      // if (s > 0) {
-      //   m_used = _dlShiftDigits(v_digits, m_used, s, v_digits);
-      // }
-      return new _Bigint(false, m_used, v_digits)._toValidInt() << s;
+      if (s > 0) {
+        m_used = _lShiftDigits(v_digits, m_used, s, v_digits);
+      }
+      return new _Bigint(false, m_used, v_digits)._toValidInt();
     }
     // No inverse if v != 1.
     var i = m_used - 1;
     while ((i > 0) && (v_digits[i] == 0)) --i;
     if ((i != 0) || (v_digits[0] != 1)) throw new RangeError("Not coprime");
 
     if (d_neg) {
       if ((d_digits[m_used] != 0) ||
           (_compareDigits(d_digits, m_used, x_digits, m_used) > 0)) {
         _absSub(d_digits, abcd_used, x_digits, m_used, d_digits);
@@ skipping 307 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);
   }
 }