README: fix broken links and BrowserClient example (#38)

- Updated links from api.dartlang.org to dartdocs.org.
- BrowserClient was missing.
- BrowserClient example: wrapped bare code in `main` function.

Author: chalin

README.md

@@ -44,20 +44,17 @@ You can also exert more fine-grained control over your requests and responses by
 creating [Request][] or [StreamedRequest][] objects yourself and passing them to
 [Client.send][].
 
-[Request]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/http/http.Request
-
-[StreamedRequest]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/http/http.StreamedRequest
-
-[Client.send]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/http/http.Client#id_send
+[Request]: https://www.dartdocs.org/documentation/http/latest/http/Request-class.html
+[StreamedRequest]: https://www.dartdocs.org/documentation/http/latest/http/StreamedRequest-class.html
+[Client.send]: https://www.dartdocs.org/documentation/http/latest/http/Client/send.html
 
 This package is designed to be composable. This makes it easy for external
 libraries to work with one another to add behavior to it. Libraries wishing to
 add behavior should create a subclass of [BaseClient][] that wraps another
 [Client][] and adds the desired behavior:
 
-[BaseClient]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/http/http.BaseClient
-
-[Client]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/http/http.Client
+[BaseClient]: https://www.dartdocs.org/documentation/http/latest/http/BaseClient-class.html
+[Client]: https://www.dartdocs.org/documentation/http/latest/http/Client-class.html
 
 ```dart
 class UserAgentClient extends http.BaseClient {
@@ -79,17 +76,20 @@ The HTTP library can be used on the browser via the [BrowserClient][] class in
 `package:http/browser_client.dart`. This client translates requests into
 XMLHttpRequests. For example:
 
+[BrowserClient]: https://www.dartdocs.org/documentation/http/latest/http.browser_client/BrowserClient-class.html
+
 ```dart
+import 'dart:async';
 import 'package:http/browser_client.dart';
-import 'package:http/http.dart' as http;
 
-var client = new BrowserClient();
-var url = "/whatsit/create";
-client.post(url, body: {"name": "doodle", "color": "blue"})
-    .then((response) {
-  print("Response status: ${response.statusCode}");
-  print("Response body: ${response.body}");
-});
+main() async {
+  var client = new BrowserClient();
+  var url = '/whatsit/create';
+  var response =
+      await client.post(url, body: {'name': 'doodle', 'color': 'blue'});
+  print('Response status: ${response.statusCode}');
+  print('Response body: ${response.body}');
+}
 ```
 
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