Fix wrong return type on fullPath in File API.

runtime/bin/file.dart

Index: runtime/bin/file.dart
diff --git a/runtime/bin/file.dart b/runtime/bin/file.dart
index 4fd8bbbfcd26655c4af433a1a671a99b4c4bcdbb..a05136b8c5253876b6e4fbe15ea17d5f6f9113c5 100644
--- a/runtime/bin/file.dart
+++ b/runtime/bin/file.dart
@@ -106,10 +106,10 @@ interface File default _File {
 
   /**
    * Get the canonical full path corresponding to the file name. The
-   * [fullPathHandler] is called when the fullPath operation
-   * completes.
+   * [fullPathHandler] is called with the result when the fullPath
+   * operation completes.
    */
-  String fullPath();
+  void fullPath();
 
   /**
    * Synchronously get the canonical full path corresponding to the file name.