Roll Observatory packages and add a roll script

packages/web_components/lib/src/normalize_path.dart

 // Copyright (c) 2015, 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 web_components.src.normalizePath;
 
 import 'package:path/path.dart' as path;
 
 String normalizeHtmlImportPath(
     String filePath, String dartFilePackage, String dartFilePath) {
   // If they already supplied a packages path, just return that.
   if (filePath.startsWith('package:')) {
     return filePath.replaceFirst('package:', 'packages/');
   }
 
   var dartFileDir = path.url.dirname(dartFilePath);
 
   // Relative paths have no package supplied.
   if (dartFilePackage == null) {
     return path.url.normalize(path.url.join(dartFileDir, filePath));
   }
 
   // Only option left is a packages/ path.
   return path.url.normalize(
       path.url.join('packages/', dartFilePackage, dartFileDir, filePath));
 }