fix links in readme

sdk/api_readme.md

 Welcome to the Dart API reference documentation, covering the official Dart API
 libraries. Some of the most fundamental Dart libraries include:
    
-  * [dart:core](dart-core/index.html): Core functionality such as strings, numbers, collections, errors, dates, and URIs.
-  * [dart:html](dart-html/index.html): DOM manipulation for web apps.
-  * [dart:io](dart-io/index.html): I/O for command-line apps.
+  * [dart:core](dart-core/dart-core-library.html): Core functionality such as strings, numbers, collections, errors, dates, and URIs.
+  * [dart:html](dart-html/dart-html-library.html): DOM manipulation for web apps.
+  * [dart:io](dart-io/dart-io-library.html): I/O for command-line apps.
   
 Except for `dart:core`, you must import a library before you can use it. Here's
 an example of importing `dart:html` and `dart:math`:
 
 ```dart
 import 'dart:html';
 import 'dart:math';
 ```
 
 You can install more libraries using the pub package manager. For information
 on finding, using, and publishing libraries with pub, see 
 [pub.dartlang.org](https://pub.dartlang.org).
   
 The main site for learning and using Dart is
 [www.dartlang.org](https://www.dartlang.org). Check out these additional pages:
   
   * [Tutorials](https://www.dartlang.org/docs/tutorials/)
   * [Programmer's Guide](https://www.dartlang.org/docs/)
   * [Samples](https://www.dartlang.org/samples/)
   * [A Tour of the Dart Libraries](https://www.dartlang.org/docs/dart-up-and-running/contents/ch03.html)
   
 This API reference is automatically generated from the source code in the
 [Dart project](https://github.com/dart-lang/sdk). If you'd like to contribute to
 this documentation, see
 [Contributing](https://github.com/dart-lang/sdk/wiki/Contributing).