Add fromUri constructor to File, Directory and Link

sdk/lib/io/link.dart

Index: sdk/lib/io/link.dart
diff --git a/sdk/lib/io/link.dart b/sdk/lib/io/link.dart
index 82aef2a5450b9f971921fd6e8a3e909fbf2c921c..7f07234ab54dc3f343009605ddbc721bc8de56f9 100644
--- a/sdk/lib/io/link.dart
+++ b/sdk/lib/io/link.dart
@@ -15,6 +15,13 @@ abstract class Link implements FileSystemEntity {
   factory Link(String path) => new _Link(path);
 
   /**
+   * Create a Lint object from a URI.
+   *
+   * If [uri] cannot reference a link this throws [UnsupportedError].
+   */
+  factory Link.fromUri(Uri uri) => new Link(uri.toFilePath());
+
+  /**
    * Creates a symbolic link. Returns a [:Future<Link>:] that completes with
    * the link when it has been created. If the link exists,
    * the future will complete with an error.
@@ -143,7 +150,6 @@ class _Link extends FileSystemEntity implements Link {
     }
   }
 
-
   String toString() => "Link: '$path'";
 
   Future<bool> exists() => FileSystemEntity.isLink(path);