use symbol literals instead of const ctor in packages

pkg/path/lib/path.dart

 // Copyright (c) 2012, 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.
 
 /// A comprehensive, cross-platform path manipulation library.
 ///
 /// ## Installing ##
 ///
 /// Use [pub][] to install this package. Add the following to your
 /// `pubspec.yaml` file.
@@ skipping 72 matching lines @@
 LibraryMirror get _html =>
   currentMirrorSystem().libraries[Uri.parse('dart:html')];
 
 /// Gets the path to the current working directory.
 ///
 /// In the browser, this means the current URL. When using dart2js, this
 /// currently returns `.` due to technical constraints. In the future, it will
 /// return the current URL.
 String get current {
   if (_io != null) {
-    return _io.classes[const Symbol('Directory')]
-        .getField(const Symbol('current')).reflectee.path;
+    return _io.classes[#Directory].getField(#current).reflectee.path;
   } else if (_html != null) {
-    return _html.getField(const Symbol('window'))
-        .reflectee.location.href;
+    return _html.getField(#window).reflectee.location.href;
   } else {
     return '.';
   }
 }
 
 /// Gets the path separator for the current platform. This is `\` on Windows
 /// and `/` on other platforms (including the browser).
 String get separator => _builder.separator;
 
 /// Converts [path] to an absolute path by resolving it relative to the current
@@ skipping 754 matching lines @@
   /// The style of the host platform.
   ///
   /// When running on the command line, this will be [windows] or [posix] based
   /// on the host operating system. On a browser, this will be [url].
   static final platform = _getPlatformStyle();
 
   /// Gets the type of the host platform.
   static Style _getPlatformStyle() {
     if (_io == null) return Style.url;
 
-    if (_io.classes[const Symbol('Platform')]
-        .getField(const Symbol('operatingSystem')).reflectee == 'windows') {
+    if (_io.classes[#Platform].getField(#operatingSystem)
+        .reflectee == 'windows') {
       return Style.windows;
     }
 
     return Style.posix;
   }
 
   /// The name of this path style. Will be "posix" or "windows".
   String get name;
 
   /// The path separator for this style. On POSIX, this is `/`. On Windows,
@@ skipping 314 matching lines @@
     // doesn't count.
     if (lastDot <= 0) return [file, ''];
 
     return [file.substring(0, lastDot), file.substring(lastDot)];
   }
 
   _ParsedPath clone() => new _ParsedPath(
       style, root, isRootRelative,
       new List.from(parts), new List.from(separators));
 }