Fix a number of editor errors and warnings in io tests

runtime/bin/file.dart

Index: runtime/bin/file.dart
diff --git a/runtime/bin/file.dart b/runtime/bin/file.dart
index ed177a8bdce30ecc40d71c18a87121afc4dce2ea..787e4b8f99f94829494a74b90ebab7b2f1e593c6 100644
--- a/runtime/bin/file.dart
+++ b/runtime/bin/file.dart
@@ -119,9 +119,11 @@ interface File default _File {
    * can be performed. Opened RandomAccessFiles must be closed using
    * the [close] method.
    *
-   * See [open] for information on the [:mode:] argument.
+   * The default value for [mode] is [:FileMode.READ:].
+   *
+   * See [open] for information on the [mode] argument.
    */
-  RandomAccessFile openSync(FileMode mode);
+  RandomAccessFile openSync([FileMode mode]);
 
   /**
    * Get the canonical full path corresponding to the file name.  The