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

tests/standalone/io/file_input_stream_test.dart

Index: tests/standalone/io/file_input_stream_test.dart
diff --git a/tests/standalone/io/file_input_stream_test.dart b/tests/standalone/io/file_input_stream_test.dart
index 8357d534f0b3594f346049b431e45e4e1f505090..923a44ccf8753454ee32488cf3cb81a94fab4e50 100644
--- a/tests/standalone/io/file_input_stream_test.dart
+++ b/tests/standalone/io/file_input_stream_test.dart
@@ -55,7 +55,7 @@ void testOpenStreamAsync() {
 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());