Move minitest.dart into the expect package.

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

 import 'dart:async';
 import 'dart:html';
 import 'dart:typed_data';
 import 'dart:web_audio';
 
-import 'package:minitest/minitest.dart';
+import 'package:expect/minitest.dart';
 
 main() {
   var isAudioContext =
       predicate((x) => x is AudioContext, 'is an AudioContext');
 
   group('supported', () {
     test('supported', () {
       expect(AudioContext.supported, true);
     });
   });
@@ skipping 69 matching lines @@
         expect(oscillator.type, equals('triangle'));
 
         // Firefox does not throw when it receives invalid values; it simply
         // ignores them.
         //expect(() => oscillator.type = ['heap object not a string'], throws);
         expect(oscillator.type, equals('triangle'));
       }
     });
   });
 }