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

sdk/lib/core/string.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 String class represents sequences of characters. Strings are
  * immutable. A string is represented by a sequence of Unicode UTF-16
  * code units accessible through the [codeUnitAt] or the
  * [codeUnits] members. Their string representation is accessible through
  * the index-operator.
  *
  * The characters of a string are encoded in UTF-16. Decoding UTF-16, which
  * combines surrogate pairs, yields Unicode code points. Following a similar
  * terminology to Go we use the name "rune" for an integer representing a
  * Unicode code point. The runes of a string are accessible through the [runes]
  * getter.
+ *
+ * It is a compile-time error for a class to attempt to extend or implement
+ * String.
  */
 abstract class String implements Comparable<String>, Pattern {
   /**
    * Allocates a new String for the specified [charCodes].
    *
    * The [charCodes] can be UTF-16 code units or runes. If a char-code value is
    * 16-bit it is copied verbatim. If it is greater than 16 bits it is
    * decomposed into a surrogate pair.
    */
   external factory String.fromCharCodes(Iterable<int> charCodes);
@@ skipping 453 matching lines @@
         _position = position - 1;
         _currentCodePoint = _combineSurrogatePair(prevCodeUnit, codeUnit);
         return true;
       }
     }
     _position = position;
     _currentCodePoint = codeUnit;
     return true;
   }
 }