Update documentation on Symbol. Make validation match.

sdk/lib/core/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.core;
 
 /// Opaque name used by mirrors, invocations and [Function.apply].
 class Symbol {
   /**
    * Constructs a new Symbol.
    *
-   * An [ArgumentError] is thrown if [name] starts with an underscore, or if
-   * [name] is not a [String].  An [ArgumentError] is thrown if [name] is not
-   * an empty string and is not a valid qualified identifier optionally
-   * followed by [:'=':].
+   * The name must be a valid public Dart member name,
+   * public constructor name, or library name, optionally qualified.
+   *
+   * A qualified name is a valid name preceded by a public identifier name
+   * and a '`.`', e.g., `foo.bar.baz=` is a qualified version of `baz=`.
+   * That means that the content of the [name] String must be either
+   *
+   * * a valid public Dart identifier
+   *   (that is, an identifier not starting with "`_`"),
+   * * such an identifier followed by "=" (a setter name),
+   * * the name of a declarable operator
+   *   (one of "`+`", "`-`", "`*`", "`/`", "`%`", "`~/`", "`&`", "`|`",
+   *   "`^`", "`~`", "`<<`", "`>>`", "`<`", "`<=`", "`>`", "`>=`", "`==`",
+   *   "`[]`", "`[]=`", or "`unary-`"),
+   * * any of the above preceeded by any number of qualifiers,
+   *   where a qualifier is a non-private identifier followed by '`.`',
+   * * or the empty string (the default name of a library with not library
+   *   name declaration).
    *
    * The following text is non-normative:
    *
    * Creating non-const Symbol instances may result in larger output.  If
    * possible, use [MirrorsUsed] in "dart:mirrors" to specify which names might
    * be passed to this constructor.
    */
   const factory Symbol(String name) = internal.Symbol;
 }