Fix pub serve on windows.

sdk/lib/_internal/pub/lib/src/command/serve.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 pub.command.serve;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:barback/barback.dart';
@@ skipping 208 matching lines @@
       // Add the initial sources.
       _barback.updateSources(listAssets(package));
 
       // Watch the visible package directories for changes.
       var packageDir = _provider.getPackageDir(package);
 
       for (var name in getPublicDirectories(package)) {
         var subdirectory = path.join(packageDir, name);
         var watcher = new DirectoryWatcher(subdirectory);
         watcher.events.listen((event) {
-          var relativePath = path.relative(event.path, from: packageDir);
-          var id = new AssetId(package, relativePath);
+          var id = pathToAssetId(package, packageDir, event.path);
           if (event.type == ChangeType.REMOVE) {
             _barback.removeSources([id]);
           } else {
             _barback.updateSources([id]);
           }
         });
       }
     }
   }
 
   /// Lists all of the visible files in [package].
   ///
   /// This is the recursive contents of the "asset" and "lib" directories (if
   /// present). If [package] is the entrypoint package, it also includes the
   /// contents of "web".
   List<AssetId> listAssets(String package) {
     var files = <AssetId>[];
 
     for (var dirPath in getPublicDirectories(package)) {
       var packageDir = _provider.getPackageDir(package);
       var dir = path.join(packageDir, dirPath);
       if (!dirExists(dir)) continue;
       for (var entry in listDir(dir, recursive: true)) {
         // Ignore "packages" symlinks if there.
         if (path.split(entry).contains("packages")) continue;
 
         // Skip directories.
         if (!fileExists(entry)) continue;
 
-        // AssetId paths use "/" on all platforms.
-        var relative = path.relative(entry, from: packageDir);
-        relative = path.toUri(relative).path;
-        files.add(new AssetId(package, relative));
+        files.add(pathToAssetId(package, packageDir, entry));
       }
     }
 
     return files;
   }
 
   /// Gets the names of the top-level directories in [package] whose contents
   /// should be provided as source assets.
   Iterable<String> getPublicDirectories(String package) {
     var directories = ["asset", "lib"];
     if (package == entrypoint.root.name) directories.add("web");
     return directories;
   }
+
+  /// Converts a local file path to an [AssetId].
+  AssetId pathToAssetId(String package, String packageDir, String filePath) {
+    var relative = path.relative(filePath, from: packageDir);
+
+    // AssetId paths use "/" on all platforms.
+    relative = path.toUri(relative).path;
+
+    return new AssetId(package, relative);
+  }
 }