Avoid UTF-8 decoding in num.toString.

runtime/lib/double.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 _Double implements double {
+class _Double extends _Num implements double {
   factory _Double.fromInteger(int value)
       native "Double_doubleFromInteger";
 
   Type get runtimeType => double;
 
   int get _identityHashCode {
     if (isNaN || isInfinite) return 0;
     return toInt();
   }
   double operator +(num other) {
@@ skipping 220 matching lines @@
         return EQUAL;
       }
     } else if (isNaN) {
       return other.isNaN ? EQUAL : GREATER;
     } else {
       // Other is NaN.
       return LESS;
     }
   }
 }