Escape slash since it helps to end an HTML entity.

tests/lib/convert/html_escape_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.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:convert';
 
 const _NOOP = 'Nothing_to_escape';
 
-const _TEST_INPUT = '<A <test> of \u00A0 "double" & \'single\' values>';
+const _TEST_INPUT = '<A </test> of \u00A0 "double" & \'single\' values>';
 
-const _OUTPUT_UNKNOWN = '&lt;A &lt;test&gt; of &nbsp; &quot;double&quot; &amp; '
-  '&apos;single&apos; values&gt;';
+const _OUTPUT_UNKNOWN = '&lt;A &lt;&#x2F;test&gt; of &nbsp; &quot;double&quot; &amp; '
+  '&#x27;single&#x27; values&gt;';
 
-const _OUTPUT_ATTRIBUTE = "<A <test> of &nbsp; &quot;double&quot; &amp; "
+const _OUTPUT_ATTRIBUTE = "<A </test> of &nbsp; &quot;double&quot; &amp; "
   "\'single\' values>";
 
-const _OUTPUT_ELEMENT = '&lt;A &lt;test&gt; of &nbsp; "double" &amp; '
+const _OUTPUT_ELEMENT = '&lt;A &lt;&#x2F;test&gt; of &nbsp; "double" &amp; '
   '\'single\' values&gt;';
 
 void _testMode(HtmlEscape escape, String input, String expected) {
   _testConvert(escape, input, expected);
   _testTransform(escape, input, expected);
   _testChunked(escape, input, expected);
 }
 
 void _testChunked(HtmlEscape escape, String input, String expected) {
   var buffer = new StringBuffer();
@@ skipping 46 matching lines @@
 const _COUNT = 3;
 
 void main() {
   _testMode(HTML_ESCAPE, _TEST_INPUT, _OUTPUT_UNKNOWN);
   _testMode(const HtmlEscape(), _TEST_INPUT, _OUTPUT_UNKNOWN);
   _testMode(const HtmlEscape(HtmlEscapeMode.UNKNOWN), _TEST_INPUT, _OUTPUT_UNKNOWN);
   _testMode(const HtmlEscape(HtmlEscapeMode.ATTRIBUTE), _TEST_INPUT, _OUTPUT_ATTRIBUTE);
   _testMode(const HtmlEscape(HtmlEscapeMode.ELEMENT), _TEST_INPUT, _OUTPUT_ELEMENT);
   _testMode(HTML_ESCAPE, _NOOP, _NOOP);
 }