Add an OutputStream.closed getter.

tests/standalone/io/file_output_stream_test.dart

 // Copyright (c) 2012, 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:io");
 #import("dart:isolate");
 
 void testOpenOutputStreamSync() {
   Directory tempDirectory = new Directory('').createTempSync();
 
   // Create a port for waiting on the final result of this test.
   ReceivePort done = new ReceivePort();
   done.receive((message, replyTo) {
     tempDirectory.deleteSync();
     done.close();
   });
 
   String fileName = "${tempDirectory.path}/test";
   File file = new File(fileName);
   file.createSync();
   OutputStream x = file.openOutputStream();
   x.write([65, 66, 67]);
+  Expect.isFalse(x.closed);
   x.close();
+  Expect.isTrue(x.closed);
   x.onClosed = () {
+    Expect.isTrue(x.closed);
     file.deleteSync();
     done.toSendPort().send("done");
   };
 }
 
 
 void testOutputStreamNoPendingWrite() {
   var tempDirectory;
 
   // Create a port for waiting on the final result of this test.
@@ skipping 58 matching lines @@
   };
   x.close();
   x.onError = (e) => Expect.fail("No error expected");
 }
 
 main() {
   testOpenOutputStreamSync();
   testOutputStreamNoPendingWrite();
   testOutputStreamFlush();
 }