Avoid runtime type errors in checked mode for ChunkedConverters.

sdk/lib/convert/html_escape.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.
 
 part of dart.convert;
 
 /**
  * A `String` converter that converts characters to HTML entities.
  *
  * This is intended to sanitice text before inserting the text into an HTML
@@ skipping 134 matching lines @@
  * * `'` (apostrophe) when inside a single-quoted attribute value.
  *       Apostrophe is escaped as `'` instead of `'` since
  *       not all browsers understand `'`.
  * * `/` (slash) is recommended to be escaped because it may be used
  *       to terminate an element in some HTML dialects.
  *
  * Escaping `>` (greater than) isn't necessary, but the result is often
  * found to be easier to read if greater-than is also escaped whenever
  * less-than is.
  */
-class HtmlEscape extends Converter<String, String> {
+class HtmlEscape extends Converter<String, String>
+    implements ChunkedConverter<String, String, String, String> {
 
   /** The [HtmlEscapeMode] used by the converter. */
   final HtmlEscapeMode mode;
 
   /**
    * Create converter that escapes HTML characters.
    *
    * If [mode] is provided as either [HtmlEscapeMode.ATTRIBUTE] or
    * [HtmlEscapeMode.ELEMENT], only the corresponding subset of HTML
    * characters are escaped.
@@ skipping 56 matching lines @@
     if(val == null) {
       _sink.addSlice(chunk, start, end, isLast);
     } else {
       _sink.add(val);
       if (isLast) _sink.close();
     }
   }
 
   void close() { _sink.close(); }
 }