VM: Remove unnecessary _Num class from runtime/lib.

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.
 
-class _IntegerImplementation extends _Num {
+class _IntegerImplementation {
   // The Dart class _Bigint extending _IntegerImplementation requires a
   // default constructor.
 
   Type get runtimeType => int;
 
   num operator +(num other) {
     var result = other._addFromInteger(this);
     if (result != null) return result;
     return other._toBigint()._addFromInteger(this);
   }
@@ skipping 599 matching lines @@
   // Shift by mint exceeds range that can be handled by the VM.
   int _shrFromInt(int other) {
     if (other < 0) {
       return -1;
     } else {
       return 0;
     }
   }
   int _shlFromInt(int other) native "Mint_shlFromInt";
 }