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

sdk/lib/core/bool.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;
 
+/**
+ * The reserved words [:true:] and [:false:] denote objects that are the only
+ * instances of this class.
+ * 
+ * It is a compile-time error for a class to attempt to extend or implement
+ * bool.
+ */
 class bool {
   factory bool._uninstantiable() {
     throw new UnsupportedError(
         "class bool cannot be instantiated");
   }
+
+  /**
+   * Returns [:"true":] if the receiver is [:true:], or [:"false":] if the

[ahe, 2013/08/15 20:02:54]

I'm not sure I like adding this method. dart2js only uses this class for an
interface, and you cannot change this method without changing implementation
details in dart2js.

[rmacnak, 2013/08/15 20:14:11]

What if I declare it to be abstract, like on int/double?

[ahe, 2013/08/15 20:15:54]

Works for me!

+   * receiver is [:false:].
+   */
+  String toString() {
+        return this ? "true" : "false";
+  }
 }