Port the remaining of dart:core to the unified corelib.

corelib/unified/core/num.dart

 // Copyright (c) 2011, 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.
 
 // Dart core library.
 
 interface num extends Comparable, Hashable {
   // Arithmetic operations.
   num operator +(num other);
   num operator -(num other);
@@ skipping 24 matching lines @@
   num truncate();
 
   int toInt();
   double toDouble();
 
   String toStringAsFixed(int fractionDigits);
   String toStringAsExponential(int fractionDigits);
   String toStringAsPrecision(int precision);
   String toRadixString(int radix);
 }