Move zero-ing code out of shift intrinsic (not needed if destination is new).

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 253 matching lines @@
     final cbs = _DIGIT_BITS - bs;
     final bm = (1 << cbs) - 1;
     var c = 0;
     var i = x_used;
     while (--i >= 0) {
       final d = x_digits[i];
       r_digits[i + ds + 1] = (d >> cbs) | c;
       c = (d & bm) << bs;
     }
     r_digits[ds] = c;
-    i = ds;
-    while (--i >= 0) {
-      r_digits[i] = 0;
-    }
   }
 
   // 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.
     _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.
     _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;
     }
     return r_used;
   }
 
   // r_digits[0..r_used-1] = x_digits[0..x_used-1] >> n.
   static void _rsh(Uint32List x_digits, int x_used, int n,
@@ skipping 1447 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);
   }
 }