Update documentation on Symbol. Make validation match.

tests/lib/mirrors/symbol_validation_test.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.
 
 library symbol_validation_test;
 
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 validSymbol(String string) {
   Expect.equals(string,
                 MirrorSystem.getName(new Symbol(string)),
                 'Valid symbol "$string" should be invertable');
   Expect.equals(string,
                 MirrorSystem.getName(MirrorSystem.getSymbol(string)),
                 'Valid symbol "$string" should be invertable');
 }
 
 invalidSymbol(String string) {
   Expect.throws(() => new Symbol(string),
                 (e) => e is ArgumentError,
                 'Invalid symbol "$string" should be rejected');
   Expect.throws(() => MirrorSystem.getSymbol(string),
                 (e) => e is ArgumentError,
                 'Invalid symbol "$string" should be rejected');
 }
 
 main() {
-  ['%', '&', '*', '+', '-', '/', '<', '<<', '<=', '==', '>',
-   '>=', '>>', '[]', '[]=', '^', 'unary-', '|', '~', '~/']
-      .forEach(validSymbol);
+  // Operators that can be declared as class member operators.
+  // These are all valid as symbols.
+  var operators = [
+    '%', '&', '*', '+', '-', '/', '<', '<<', '<=', '==', '>',
+    '>=', '>>', '[]', '[]=', '^', 'unary-', '|', '~', '~/'
+  ];
+  operator.forEach(validSymbol);
 
-  ['foo', '_bar', 'baz.quz', 'fisk1', 'hest2fisk', 'a.b.c.d.e',
-   '\$', 'foo\$', 'bar\$bar']
-      .forEach(validSymbol);
+  var simpleSymbols = [
+    'foo', '_bar', 'baz.quz', 'fisk1', 'hest2fisk', 'a.b.c.d.e',
+    r'$', r'foo$', r'bar$bar', r'$.$', r'x6$_', r'$6_', r'x.$$6_',
+    'x_', 'x_.x_',
+  ];
+  simpleSymbols.forEach(validSymbol);
 
-  ['6', '0foo', ',', 'S with M', '_invalid&private']
-      .forEach(invalidSymbol);
+  var nonSymbols = [
+    // Non-identifiers.
+    '6', '0foo', ',', 'S with M', '_invalid&private', "#foo", " foo", "foo ",
+    // Operator variants.
+    '+=', '()', 'operator+', 'unary+', '>>>', "&&", "||", "!", "@", "#", "[",
+    // Privat symbols.
+    '_x', 'x._y', 'x._',
+    // Empty parts of "qualified" symbols.
+    '.', 'x.', '.x', 'x..y'
+  ];
+  nonSymbols.forEach(invalidSymbol);
+
+  // Reserved words are not valid identifiers and therefore not valid symbols.
+  var reservedWords = [
+    "assert",
+    "break",
+    "case",
+    "catch",
+    "class",
+    "const",
+    "continue",
+    "default",
+    "do",
+    "else",
+    "enum",
+    "extends",
+    "false",
+    "final",
+    "finally",
+    "for",
+    "if",
+    "in",
+    "is",
+    "new",
+    "null",
+    "rethrow",
+    "return",
+    "super",
+    "switch",
+    "this",
+    "throw",
+    "true",
+    "try",
+    "var",
+    "void",
+    "while",
+    "with"
+  ];
+  reservedWords.expand((w) => [w, "$w=", "x.$w" , "$w.x", "x.$w.x"])
+               .forEach(invalidSymbol);
+
+  // Built-in identifiers are valid identifiers that are restricted from being
+  // used in some cases, but they are all valid symbols.
+  var builtInIdentifiers = [
+    "abstract",
+    "as",
+    "dynamic",
+    "export",
+    "external",
+    "factory",
+    "get",
+    "implements",
+    "import",
+    "library",
+    "operator",
+    "part",
+    "set",
+    "static",
+    "typedef"
+  ];
+  builtInIdentifiers.expand((w) => [w, "$w=", "x.$w" , "$w.x", "x.$w.x"])
+                    .forEach(validSymbol);
 }