Removed fixed dependencies

packages/usage/lib/usage_html.dart

 // Copyright (c) 2014, 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.
 
-/// In order to use this library import the `usage_html.dart` file and
-/// instantiate the [AnalyticsHtml] class.
-///
-/// You'll need to provide a Google Analytics tracking ID, the application name,
-/// and the application version.
+/**
+ * In order to use this library import the `usage_html.dart` file and
+ * instantiate the [AnalyticsHtml] class.
+ *
+ * You'll need to provide a Google Analytics tracking ID, the application name,
+ * and the application version.
+ */
 library usage_html;
 
-export 'src/usage_impl_html.dart' show AnalyticsHtml;
+import 'dart:html';
+
+import 'src/usage_impl.dart';
+import 'src/usage_impl_html.dart';
+
 export 'usage.dart';
+
+/**
+ * An interface to a Google Analytics session, suitable for use in web apps.
+ *
+ * [analyticsUrl] is an optional replacement for the default Google Analytics
+ * URL (`https://www.google-analytics.com/collect`).
+ */
+class AnalyticsHtml extends AnalyticsImpl {
+  AnalyticsHtml(String trackingId, String applicationName, String applicationVersion, {
+    String analyticsUrl
+  }) : super(
+      trackingId,
+      new HtmlPersistentProperties(applicationName),
+      new HtmlPostHandler(),
+      applicationName: applicationName,
+      applicationVersion: applicationVersion,
+      analyticsUrl: analyticsUrl
+  ) {
+    int screenWidth = window.screen.width;
+    int screenHeight = window.screen.height;
+
+    setSessionValue('sr', '${screenWidth}x$screenHeight');
+    setSessionValue('sd', '${window.screen.pixelDepth}-bits');
+    setSessionValue('ul', window.navigator.language);
+  }
+}