Avoid UTF-8 decoding in num.toString.

runtime/lib/integers.dart

 // Copyright (c) 2012, 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.
 
 // TODO(srdjan): fix limitations.
 // - shift amount must be a Smi.
-class _IntegerImplementation {
+class _IntegerImplementation extends _Num {
   factory _IntegerImplementation._uninstantiable() {
     throw new UnsupportedError(
         "_IntegerImplementation can only be allocated by the VM");
   }
 
   Type get runtimeType => int;
 
   num operator +(num other) {
     return other._addFromInteger(this);
   }
@@ skipping 449 matching lines @@
     } else {
       return 0;
     }
   }
   int _shlFromInt(int other) native "Bigint_shlFromInt";
 
   int pow(int exponent) {
     throw "Bigint.pow not implemented";
   }
 }