Move minitest.dart into the expect package.

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

 import 'dart:html';
 
-import 'package:minitest/minitest.dart';
+import 'package:expect/minitest.dart';
 
 main() {
   test('EventException', () {
     final event = new Event('Event');
     // Intentionally do not initialize it!
     try {
       document.dispatchEvent(event);
     } on DomException catch (e) {
       expect(e.name, DomException.INVALID_STATE);
     }
   });
 }