Eliminate unnecessary factory constructors in abstract core classes

lib/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.
 
 abstract class num implements Comparable, Hashable {
-  factory num._uninstantiable() {
-    throw const UnsupportedOperationException(
-        "abstract class num cannot be instantiated");
-  }
   // Arithmetic operations.
   abstract num operator +(num other);
   abstract num operator -(num other);
   abstract num operator *(num other);
   abstract num operator %(num other);
   abstract double operator /(num other);
   // Truncating division.
   abstract num operator ~/(num other);
   // The unary '-' operator.
   abstract num operator negate();
@@ skipping 17 matching lines @@
   abstract num truncate();
 
   abstract int toInt();
   abstract double toDouble();
 
   abstract String toStringAsFixed(int fractionDigits);
   abstract String toStringAsExponential(int fractionDigits);
   abstract String toStringAsPrecision(int precision);
   abstract String toRadixString(int radix);
 }