Move minitest.dart into the expect package.

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

 import 'dart:html';
 
-import 'package:minitest/minitest.dart';
+import 'package:expect/minitest.dart';
 
 main() {
   test('constructorTest1', () {
       var audio = new AudioElement();
       expect(audio, isNotNull);
       expect(audio is AudioElement, isTrue);
     });
 
   test('constructorTest2', () {
       var audio = new AudioElement('IntentionallyMissingFileURL');
       expect(audio, isNotNull);
       expect(audio is AudioElement, isTrue);
       expect(audio.src.contains('IntentionallyMissingFileURL'), isTrue);
     });
 
   test('canPlayTypeTest', () {
       var audio = new AudioElement();
       var canPlay = audio.canPlayType("audio/mp4");
       expect(canPlay, isNotNull);
       expect(canPlay is String, isTrue);
     });
 }