Update dart:mirrors to use Symbol.

dart/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 identifier
-   * optionally followed by [:'=':].
+   * [name] is not an empty string and is not a valid qualified
+   * identifier optionally followed by [:'=':].
    */
-  external const Symbol(String name);
-
-  external bool operator ==(other);
-
-  external int get hashCode;
+  const factory Symbol(String name) = _collection_dev.Symbol;
 }