pkg/js: improved readme, added example

pkg/js/example/chart.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 chart;
+
+import 'dart:html';
+import 'package:js/js.dart';
+
+@Js()
+class Chart {
+  external Chart(CanvasRenderingContext2D ctx);
+
+  external dynamic Line(Data data, Options options);
+}
+
+@Js()
+class Data {
+  external List get labels;
+  external List<DataSet> get datasets;
+
+  external factory Data({List<String> labels, List<DataSet> datasets});
+}
+
+/// Minimal implementation of dataset for line chart
+///
+/// http://www.chartjs.org/docs/#line-chart-data-structure
+@Js()
+class DataSet {
+  external String get label;
+  external String get fillColor;
+  external String get strokeColor;
+  external String get pointColor;
+  external String get pointStrokeColor;
+  external String get pointHighlightFill;
+  external String get pointHighlightStroke;
+
+  external List<num> get data;
+
+  external factory DataSet(
+      {String label,
+      String fillColor,
+      String strokeColor,
+      String pointColor,
+      String pointStrokeColor,
+      String pointHighlightFill,
+      String pointHighlightStroke,
+      List<num> data});
+}
+
+/// Minimal implementation of options
+///
+/// http://www.chartjs.org/docs/#getting-started-global-chart-configuration
+@Js()
+class Options {
+  external bool get responsive;
+
+  external factory Options({bool responsive});
+}