Roll Observatory packages and add a roll script

web_components/lib/html_import_annotation.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.html_import_annotation;
-
-import 'dart:async';
-import 'dart:html';
-import 'package:initialize/initialize.dart';
-import 'src/normalize_path.dart';
-
-/// Annotation for a dart library which injects an html import into the
-/// current html document. The imported file must not contain any dart script
-/// tags, as they cannot be dynamically loaded.
-class HtmlImport implements Initializer<LibraryIdentifier> {
-  final String filePath;
-
-  const HtmlImport(this.filePath);
-
-  Future initialize(LibraryIdentifier library) {
-    var element = new LinkElement()
-      ..rel = 'import'
-      ..href = normalizeHtmlImportPath(filePath, library.package, library.path);
-    document.head.append(element);
-    var completer = new Completer();
-    var listeners = [
-      element.on['load'].listen((_) => completer.complete()),
-      element.on['error'].listen((_) {
-        print('Error loading html import from path `$filePath`');
-        completer.complete();
-      }),
-    ];
-    return completer.future.then((_) {
-      listeners.forEach((listener) => listener.cancel());
-    });
-  }
-}