Implement File.lastModified.

runtime/bin/file.dart

Index: runtime/bin/file.dart
diff --git a/runtime/bin/file.dart b/runtime/bin/file.dart
index 248e4b4595d77ee21ee10b8e3bfbde53efb70718..f55bc302ad7e58cb5144a83e7acc2df0227c70bb 100644
--- a/runtime/bin/file.dart
+++ b/runtime/bin/file.dart
@@ -92,6 +92,19 @@ interface File default _File {
   int lengthSync();
 
   /**
+   * Get the last-modified time of the file. Returns a
+   * [:Future<Date>:] that completes with a [Date] object for the
+   * modification date.
+   */
+  Future<Date> lastModified();
+
+  /**
+   * Get the last-modified time of the file. Throws an exception
+   * if the file does not exist.
+   */
+  Date lastModifiedSync();
+
+  /**
    * Open the file for random access operations. Returns a
    * [:Future<RandomAccessFile>:] that completes with the opened
    * random access file. RandomAccessFiles must be closed using the