VM: Remove unnecessary _Num class from runtime/lib.

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 extends _Num implements double {
+class _Double 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 117 matching lines @@
   num _toBigintOrDouble() { return this; }
   double toDouble() { return this; }
 
   static const int CACHE_SIZE_LOG2 = 3;
   static const int CACHE_LENGTH = 1 << (CACHE_SIZE_LOG2 + 1);
   static const int CACHE_MASK = CACHE_LENGTH - 1;
   // Each key (double) followed by its toString result.
   static final List _cache = new List(CACHE_LENGTH);
   static int _cacheEvictIndex = 0;
 
+  String _toString() native "Double_toString";
+
   String toString() {
     // TODO(koda): Consider starting at most recently inserted.
     for (int i = 0; i < CACHE_LENGTH; i += 2) {
       // Need 'identical' to handle negative zero, etc.
       if (identical(_cache[i], this)) {
         return _cache[i + 1];
       }
     }
     // TODO(koda): Consider optimizing all small integral values.
     if (identical(0.0, this)) {
       return "0.0";
     }
-    String result = super.toString();
+    String result = _toString();
     // Replace the least recently inserted entry.
     _cache[_cacheEvictIndex] = this;
     _cache[_cacheEvictIndex + 1] = result;
     _cacheEvictIndex = (_cacheEvictIndex + 2) & CACHE_MASK;
     return result;
   }
 
   String toStringAsFixed(int fractionDigits) {
     // See ECMAScript-262, 15.7.4.5 for details.
 
@@ skipping 94 matching lines @@
         return EQUAL;
       }
     } else if (isNaN) {
       return other.isNaN ? EQUAL : GREATER;
     } else {
       // Other is NaN.
       return LESS;
     }
   }
 }