Bug cleanup: unify on allowing radices from 2 to 36 in

lib/core/num.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// 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.
 
 // Dart core library.
 
 abstract class num implements Comparable, Hashable {
   // Arithmetic operations.
   num operator +(num other);
   num operator -(num other);
   num operator *(num other);
@@ skipping 21 matching lines @@
   num floor();
   num ceil();
   num truncate();
 
   int toInt();
   double toDouble();
 
   String toStringAsFixed(int fractionDigits);
   String toStringAsExponential(int fractionDigits);
   String toStringAsPrecision(int precision);
+
+  /**
+   * Converts a [num] to a string representation in the given [radix].
+   *
+   * The [num] in converted to an [int] using [toInt]. That [int] is
+   * then converted to a string representation with the given
+   * [radix]. In the string representation, lower-case letters are
+   * used for digits above '9'.
+   *
+   * The [radix] argument must be an integer between 2 and 36.
+   */
   String toRadixString(int radix);
 }