Add fromUri constructor to File, Directory and Link

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 1c0e37142e8899b19ff2a8a1bcb70a2425a76044..db7c924f1963de86bba1919fb81208b240c1af5f 100644
--- a/sdk/lib/io/file.dart
+++ b/sdk/lib/io/file.dart
@@ -41,6 +41,13 @@ abstract class File implements FileSystemEntity {
   factory File(String path) => new _File(path);
 
   /**
+   * Create a File object from a URI.
+   *
+   * If [uri] cannot reference a file this throws [UnsupportedError].
+   */
+  factory File.fromUri(Uri uri) => new File(uri.toFilePath());
+
+  /**
    * Create the file. Returns a [:Future<File>:] that completes with
    * the file when it has been created.
    *