Roll Observatory packages and add a roll script

packages/usage/example/example.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.
+
+/**
+ * A simple web app to hand-test the usage library.
+ */
+library usage_example;
+
+import 'dart:html';
+
+import 'package:usage/usage_html.dart';
+
+Analytics _analytics;
+String _lastUa;
+int _count = 0;
+
+void main() {
+  querySelector('#foo').onClick.listen((_) => _handleFoo(getAnalytics()));
+  querySelector('#bar').onClick.listen((_) => _handleBar(getAnalytics()));
+  querySelector('#page').onClick.listen((_) => _changePage(getAnalytics()));
+}
+
+String _ua() => (querySelector('#ua') as InputElement).value.trim();
+
+Analytics getAnalytics() {
+  if (_analytics == null || _lastUa != _ua()) {
+    _lastUa = _ua();
+    _analytics = new AnalyticsHtml(_lastUa, 'Test app', '1.0');
+    _analytics.sendScreenView(window.location.pathname);
+  }
+
+  return _analytics;
+}
+
+void _handleFoo(Analytics analytics) {
+  analytics.sendEvent('main', 'foo');
+}
+
+void _handleBar(Analytics analytics) {
+  analytics.sendEvent('main', 'bar');
+}
+
+void _changePage(Analytics analytics) {
+  window.history.pushState(null, 'new page', '${++_count}.html');
+  analytics.sendScreenView(window.location.pathname);
+}