dart:io | Add Link class, as sibling to File and Directory.

sdk/lib/io/file.dart

Index: sdk/lib/io/file.dart
diff --git a/sdk/lib/io/file.dart b/sdk/lib/io/file.dart
index 1bfcb6cc42022f203a06db3c71036ef5a321d106..d484cf382d7c684c3f498a42519df645b52ae542 100644
--- a/sdk/lib/io/file.dart
+++ b/sdk/lib/io/file.dart
@@ -19,6 +19,11 @@ class FileMode {
 /**
  * [File] objects are references to files.
  *
+ * If [path] is a symbolic link, rather than a file, then
+ * the methods of [File] operate on the ultimate target of the
+ * link, except for File.delete and File.deleteSync, which operate on
+ * the link.
+ *
  * To operate on the underlying file data there are two options:
  *
  *  * Use streaming: read the contents of the file from the [Stream]