Roll Observatory packages and add a roll script

packages/web_components/lib/build/common.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.build.common;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:barback/barback.dart';
 import 'package:code_transformers/messages/build_logger.dart';
 import 'package:html/dom.dart' as dom;
 import 'package:html/parser.dart';
@@ skipping 58 matching lines @@
 
 /// Parses an HTML file [contents] and returns a DOM-like tree.
 dom.Document parseHtml(String contents, String sourcePath) {
   var parser = new HtmlParser(contents,
       encoding: 'utf8', generateSpans: true, sourceUrl: sourcePath);
   return parser.parse();
 }
 
 const String dartType = "application/dart";
 const String jsType = "text/javascript";