Make html-escape also escape < and > in attributes.

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 \xA0 "double" & 'single' values>""";
 
 const _OUTPUT_UNKNOWN = '&lt;A &lt;&#47;test&gt; of \xA0 &quot;double&quot; '
   '&amp; &#39;single&#39; values&gt;';
 
 const _OUTPUT_ATTRIBUTE =
-    "<A </test> of \xA0 &quot;double&quot; &amp; 'single' values>";
+    "&lt;A &lt;/test&gt; of \xA0 &quot;double&quot; &amp; 'single' values&gt;";
 
 const _OUTPUT_SQ_ATTRIBUTE =
-    '<A </test> of \xA0 "double" &amp; &#39;single&#39; values>';
+    '&lt;A &lt;/test&gt; of \xA0 "double" &amp; &#39;single&#39; values&gt;';
 
 const _OUTPUT_ELEMENT =
     """&lt;A &lt;/test&gt; of \xA0 "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);
 }
 
@@ skipping 48 matching lines @@
 
 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.SQ_ATTRIBUTE), _TEST_INPUT, _OUTPUT_SQ_ATTRIBUTE);
   _testMode(const HtmlEscape(HtmlEscapeMode.ELEMENT), _TEST_INPUT, _OUTPUT_ELEMENT);
   _testMode(HTML_ESCAPE, _NOOP, _NOOP);
 }