Remove uses of unittest in the HTML tests where possible.

pkg/dev_compiler/test/codegen/lib/html/mediasource_test.dart

-library mediasource_test;
-
-import 'package:unittest/unittest.dart';
-import 'package:unittest/html_individual_config.dart';
 import 'dart:html';
 import 'dart:typed_data';
-import 'dart:async';
+
+import 'package:expect/minitest.dart';
 
 main() {
-  useHtmlIndividualConfiguration();
-
   var isMediaSource = predicate((x) => x is MediaSource, 'is a MediaSource');
 
   group('supported', () {
     test('supported', () {
       expect(MediaSource.supported, true);
     });
   });
 
   // TODO(alanknight): Actually exercise this, right now the tests are trivial.
   group('functional', () {
@@ skipping 11 matching lines @@
 
     test('media types', () {
       if (MediaSource.supported) {
         expect(MediaSource.isTypeSupported('text/html'), false);
         expect(MediaSource.isTypeSupported('video/webm;codecs="vp8,vorbis"'),
             true);
       }
     });
   });
 }