Add 'start' and 'end' optional arguments to File.openRead(). This makes it possible to stream a sub…

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 20110af9ec064796cabd265f41b49323b484cf0d..eb3dcd7c8e6f25744b558646e9b0ff1adde0e443 100644
--- a/sdk/lib/io/file.dart
+++ b/sdk/lib/io/file.dart
@@ -180,14 +180,19 @@ abstract class File extends FileSystemEntity {
   String fullPathSync();
 
   /**
-   * Create a new independent [Stream](../dart_async/Stream.html) for the
-   * contents of this file.
+   * Create a new independent [Stream] for the contents of this file.
+   *
+   * If [start] is present, the file will be read from byte-offset [start].
+   * Otherwise from the beginning (index 0).
+   *
+   * If [end] is present, only up to byte-index [end] will be read. Otherwise,
+   * until end of file.
    *
    * In order to make sure that system resources are freed, the stream
    * must be read to completion or the subscription on the stream must
    * be cancelled.
    */
-  Stream<List<int>> openRead();
+  Stream<List<int>> openRead([int start, int end]);
 
   /**
    * Creates a new independent [IOSink] for the file. The