Modified unittest to use new argument syntax.

tests/html/audiobuffersourcenode_test.dart

 #library('AudioBufferSourceNodeTest');
 #import('../../pkg/unittest/unittest.dart');
 #import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
 
 main() {
 
   useHtmlConfiguration();
 
   test('createBuffer', () {
       var ctx = new AudioContext();
       AudioBufferSourceNode node = ctx.createBufferSource();
       node.start(ctx.currentTime, 0, 2);
       node.stop(ctx.currentTime + 2);
-      expect(node is AudioBufferSourceNode);
+      expect(node is AudioBufferSourceNode, isTrue);
   });
 }