Update dart:mirrors to use Symbol.

dart/sdk/lib/_collection_dev/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._collection.dev;
+
+/**
+ * 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;
+
+  external const Symbol(String name);
+
+  /**
+   * Platform-private method used by the mirror system to create
+   * otherwise invalid names.
+   */
+  const Symbol.unvalidated(this._name);
+
+  bool operator ==(other) => other is Symbol && _name == other._name;
+
+  int get hashCode {
+    const arbitraryPrime = 664597;
+    return 0x1fffffff & (arbitraryPrime * _name.hashCode);
+  }
+
+  /// Platform-private accessor which cannot be called from user libraries.
+  static String getName(Symbol symbol) => symbol._name;
+}