Fix some minor static type issues in dart:io implementation.

runtime/bin/file_impl.dart

Index: runtime/bin/file_impl.dart
diff --git a/runtime/bin/file_impl.dart b/runtime/bin/file_impl.dart
index 479cea36b0a2fd12c872def397baff59ed31c39c..70af2ac491dfa79bcf107b8026aa19f1d133e496 100644
--- a/runtime/bin/file_impl.dart
+++ b/runtime/bin/file_impl.dart
@@ -489,7 +489,7 @@ class _File implements File {
     });
   }
 
-  RandomAccessFile openSync([FileMode mode = FileMode.READ]) {
+  RandomAccessFile openSync(FileMode mode) {

[Bill Hesse, 2012/03/14 15:32:42]

Removed.

The openSync() method is called with no arguments at about 30 spots, scattered
throughout the Dart code base.  These must all be changed, if we make the mode
argument non-optional.  This should be done in a separate change, if at all.

     if (_asyncUsed) {
       throw new FileIOException(
           "Mixed use of synchronous and asynchronous API");
@@ -572,7 +572,7 @@ class _File implements File {
       throw new FileIOException(
           "Mixed use of synchronous and asynchronous API");
     }
-    var opened = openSync();
+    var opened = openSync(FileMode.READ);
     var length = opened.lengthSync();
     var result = new ByteArray(length);
     var read = opened.readListSync(result, 0, length);
@@ -655,7 +655,7 @@ class _File implements File {
 
   String get name() => _name;
 
-  void set onError(void handler(String error)) {
+  void set onError(void handler(Exception error)) {
     _onError = handler;
   }