Add an OutputStream.closed getter.

compiler/javatests/com/google/dart/compiler/parser/StringBuffer.dart

 // Copyright (c) 2011, 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.
 
 /**
  * The StringBuffer class is useful for concatenating strings
  * efficiently. Only on a call to [toString] are the strings
  * concatenated to a single String.
  */
 class StringBuffer implements OutputStream {
@@ skipping 67 matching lines @@
     String result = StringBase.concatAll(_buffer);
     buffer_.clear();
     buffer_.add(result);
     // Since we track the length at each append operation, there is no
     // need to update it in this function.
     return result;
   }
 
   GrowableArray<String> buffer_;
   int length_;
+  final bool closed = false;
 }