Improve DartDocs for dart:core classes that may not be extended or implemented.

sdk/lib/core/int.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.
 
 part of dart.core;
 
 /**
  * Representation of Dart integers containing integer specific
  * operations and specialization of operations inherited from [num].
  *
  * Integers can be arbitrarily large in Dart.
  *
  * *Note however, that when compiling to JavaScript, integers are
  * implemented as JavaScript numbers. When compiling to JavaScript,
  * integers are therefore restricted to 53 significant bits because
  * all JavaScript numbers are double-precision floating point
  * values. The behavior of the operators and methods in the [int]
  * class therefore sometimes differs between the Dart VM and Dart code
  * compiled to JavaScript.*
+ *
+ * It is a compile-time error for a class to attempt to extend or implement int.
  */
 abstract class int extends num {
   /** The bit-wise and operator. */
   int operator &(int other);
 
   /** The bit-wise or operator. */
   int operator |(int other);
 
   /** The bit-wise xor operator. */
   int operator ^(int other);
@@ skipping 82 matching lines @@
    * value is used instead. If no [onError] is provided, a [FormatException]
    * is thrown.
    *
    * The [onError] function is only invoked if [source] is a [String]. It is
    * not invoked if the [source] is, for example, `null`.
    */
   external static int parse(String source,
                             { int radix,
                               int onError(String source) });
 }