Update unittest to new package layout.

tests/html/audiocontext_test.dart

 #library('AudioContextTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
+#import('../../pkg/unittest/lib/unittest.dart');
+#import('../../pkg/unittest/lib/html_config.dart');
 #import('dart:html');
 
 main() {
 
   useHtmlConfiguration();
 
   test('constructorTest', () {
       var ctx = new AudioContext();
       expect(ctx, isNotNull);
       expect(ctx is AudioContext);
   });
   test('createBuffer', () {
       var ctx = new AudioContext();
       ArrayBufferView arrayBufferView = new Float32Array.fromList([]);
       try {
         // Test that native overload is chosen correctly. Native implementation
         // should throw 'SYNTAX_ERR' DOMException because the buffer is empty.
         AudioBuffer buffer = ctx.createBuffer(arrayBufferView.buffer, false);
       } catch (e) {
         expect(e.code, equals(DOMException.SYNTAX_ERR));
       }
   });
 }