class-level docs for several dart:io classes

sdk/lib/io/directory.dart

Index: sdk/lib/io/directory.dart
diff --git a/sdk/lib/io/directory.dart b/sdk/lib/io/directory.dart
index c23307f01f98ecb66c95d9d929837e891c17b115..e977d2fe86b0cf2966c6f498b179f77e69ab72ba 100644
--- a/sdk/lib/io/directory.dart
+++ b/sdk/lib/io/directory.dart
@@ -6,6 +6,110 @@ part of dart.io;
 
 /**
  * A reference to a directory (or _folder_) on the file system.
+ *
+ * A Directory instance is an object holding a [path] on which operations can
+ * be performed. The path to the directory can be [absolute] or [relative].
+ * You can get the parent directory using the getter [parent],
+ * a property inherited from [FileSystemEntity].
+ *
+ * In addition to being used as an instance to access the file system,
+ * Directory has a number of static properties, such as [systemTemp],
+ * which gets the system's temporary directory, and the getter and setter
+ * [current], which you can use to access or change the current directory.
+ *
+ * Create a new Directory object with a pathname to access the specified
+ * directory on the file system from your program.
+ *
+ *     var myDir = new Directory('myDir');
+ *
+ * Most methods in this class occur in synchronous and asynchronous pairs,
+ * for example, [create] and [createSync].
+ * Unless you have a specific reason for using the synchronous version
+ * of a method, prefer the asynchronous version to avoid blocking your program.
+ *
+ * ## Create a directory
+ *
+ * The following code sample creates a directory using the [create] method.
+ * By setting the `recursive` parameter to true, you can create the
+ * named directory and all its necessary parent directories,
+ * if they do not already exist.
+ *
+ *     import 'dart:io';
+ *
+ *     void main() {
+ *       // Creates dir/ and dir/subdir/.
+ *       new Directory('dir/subdir').create(recursive: true)
+ *         // The created directory is returned as a Future.
+ *         .then((Directory directory) {
+ *           print(directory.path);
+ *       });
+ *     }
+ * 
+ * ## List a directory
+ *
+ * Use the [list] or [listSync] methods to get the files and directories
+ * contained by a directory.
+ * Set `recursive` to true to recursively list all subdirectories.
+ * Set `followLinks` to true to follow symbolic links.
+ * The list method returns a [Stream] that provides FileSystemEntity
+ * objects. Use the listen callback function to process each object
+ * as it become available.
+ *
+ *     import 'dart:io';
+ *
+ *     void main() {
+ *       // Get the system temp directory.
+ *       var systemTempDir = Directory.systemTemp;
+ *
+ *       // List directory contents, recursing into sub-directories,
+ *       // but not following symbolic links.
+ *       systemTempDir.list(recursive: true, followLinks: false)
+ *         .listen((FileSystemEntity entity) {
+ *           print(entity.path);
+ *         });
+ *     }
+ *
+ * ## The use of Futures
+ *
+ * I/O operations can block a program for some period of time while it waits for
+ * the operation to complete. To avoid this, all
+ * methods involving I/O have an asynchronous variant which returns a [Future].
+ * This future completes when the I/O operation finishes. While the I/O
+ * operation is in progress, the Dart program is not blocked,
+ * and can perform other operations.
+ *
+ * For example,
+ * the [exists] method, which determines whether the directory exists,
+ * returns a boolean value using a Future.
+ * Use `then` to register a callback function, which is called when
+ * the value is ready.
+ *
+ *     import 'dart:io';
+ *
+ *     main() {
+ *       final myDir = new Directory('dir');
+ *       myDir.exists().then((isThere) {
+ *         isThere ? print('exists') : print('non-existent');
+ *       });
+ *     }
+ *
+ *
+ * In addition to exists, the [stat], [rename], and
+ * other methods, return Futures.
+ *
+ * ## Other resources
+ *
+ * * [Dart by Example](https://www.dartlang.org/dart-by-example/#files-directories-and-symlinks)
+ * provides additional task-oriented code samples that show how to use 
+ * various API from the Directory class and the related [File] class.
+ *
+ * * [I/O for Command-Line Apps](https://www.dartlang.org/docs/dart-up-and-running/contents/ch03.html#ch03-dartio---file-and-socket-io-for-command-line-apps)
+ * a section from _A Tour of the Dart Libraries_
+ * covers files and directories.
+ *
+ * * [Write Command-Line Apps](https://www.dartlang.org/docs/tutorials/cmdline/),
+ * a tutorial about writing command-line apps, includes information
+ * about files and directories.
  */
 abstract class Directory extends FileSystemEntity {
   /**