fix a test

Author: devoncarew

lib/templates/_source_code.html

@@ -1,5 +1,5 @@
 {{#hasSourceCode}}
 <section class="summary source-code" id="source">
   <h2><span>Source</span> {{{crossdartHtmlTag}}}</h2>
-  <pre class="prettyprint lang-dart">{{{ sourceCode }}}</pre>
+  <pre class="prettyprint language-dart">{{{ sourceCode }}}</pre>
 </section>{{/hasSourceCode}}

test/model_test.dart

@@ -401,7 +401,7 @@ void main() {
     test('no references', () {
       expect(
           Apple.documentationAsHtml,
-          '<p>Sample class <code>String</code></p><pre class="prettyprint lang-dart">  A\n'
+          '<p>Sample class <code>String</code></p><pre class="prettyprint language-dart">  A\n'
           '   B\n'
           '</pre>');
     });