Update documentation on Symbol. Make validation match.

sdk/lib/internal/symbol.dart

 // Copyright (c) 2013, 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._internal;
 
 /**
  * Implementation of [core.Symbol].  This class uses the same name as
  * a core class so a user can't tell the difference.
  *
  * The purpose of this class is to hide [_name] from user code, but
  * make it accessible to Dart platform code via the static method
  * [getName].
  */
 class Symbol implements core.Symbol {
   final String _name;
 
+  // Reserved words are not allowed as identifiers.
+  static const String reservedWord =
+      r'assert|break|c(?:a(?:se|tch)|lass|on(?:st|tinue))|d(?:efault|o)|'
+      r'e(?:lse|num|xtends)|f(?:alse|inal(?:ly)?|or)|i[fns]|n(?:ew|ull)|'
+      r'ret(?:hrow|urn)|s(?:uper|witch)|t(?:h(?:is|row)|r(?:ue|y))|'
+      r'v(?:ar|oid)|w(?:hile|ith)';
+
   static final RegExp validationPattern =
-      new RegExp(r'^(?:[a-zA-Z$][a-zA-Z$0-9_]*\.)*(?:[a-zA-Z$][a-zA-Z$0-9_]*=?|'
-                 r'-|'
+      new RegExp(r'^(?:'
+                 r'[\-+*/%&|^]|'
+                 r'\[\]=?|'
+                 r'==|'
+                 r'~/?|'
+                 r'<[<=]?|'
+                 r'>[>=]?|'
                  r'unary-|'
-                 r'\[\]=|'
-                 r'~|'
-                 r'==|'
-                 r'\[\]|'
-                 r'\*|'
-                 r'/|'
-                 r'%|'
-                 r'~/|'
-                 r'\+|'
-                 r'<<|'
-                 r'>>|'
-                 r'>=|'
-                 r'>|'
-                 r'<=|'
-                 r'<|'
-                 r'&|'
-                 r'\^|'
-                 r'\|'
+                 r'(?!(?:''$reservedWord'r')\b)[a-zA-Z$][\w$]*'
+                 r'(?:=?|(?:\.(?!(?:''$reservedWord'r')\b)[a-zA-Z$][\w$]*)*)'
                  r')$');
 
   external const Symbol(String name);
 
   /**
    * Platform-private method used by the mirror system to create
    * otherwise invalid names.
    */
   const Symbol.unvalidated(this._name);
 
@@ skipping 18 matching lines @@
     if (name.startsWith('_')) {
       throw new ArgumentError('"$name" is a private identifier');
     }
     if (!validationPattern.hasMatch(name)) {
       throw new ArgumentError(
           '"$name" is not an identifier or an empty String');
     }
     return name;
   }
 }