Make File:openRead always have one outstanding read, when invoking callbacks.

tests/standalone/io/file_input_stream_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // Testing file input stream, VM-only, standalone test.
 
 import "dart:convert";
 import "dart:io";
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
@@ skipping 37 matching lines @@
         asyncEnd();
       });
 }
 
 
 // Create a file that is big enough that a file stream will
 // read it in multiple chunks.
 int writeLongFileSync(File file) {
   file.createSync();
   StringBuffer buffer = new StringBuffer();
-  for (var i = 0; i < 10000; i++) {
+  for (var i = 0; i < 50000; i++) {
     buffer.write("Hello, world");
   }
   file.writeAsStringSync(buffer.toString());
   var length = file.lengthSync();
   Expect.equals(buffer.length, length);
   return length;
 }
 
 
 void testInputStreamTruncate() {
@@ skipping 187 matching lines @@
   testInputStreamDelete();
   testInputStreamAppend();
   testInputStreamOffset();
   testInputStreamBadOffset();
   // Check the length of these files as both are text files where one
   // is without a terminating line separator which can easily be added
   // back if accidentally opened in a text editor.
   testStringLineSplitterEnding("readline_test1.dat", 111);
   testStringLineSplitterEnding("readline_test2.dat", 114);
 }