Move minitest.dart into the expect package.

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

 import 'dart:html';
 
-import 'package:minitest/minitest.dart';
+import 'package:expect/minitest.dart';
 
 void check(InputElement element, String type, [bool supported = true]) {
   expect(element is InputElement, true);
   if (supported) {
     expect(element.type, type);
   } else {
     expect(element.type, 'text');
   }
 }
 
@@ skipping 169 matching lines @@
     });
     test('valueSetNullProxy', () {
       final e = new TextInputElement();
       e.value = _undefined;
       expect(e.value, '');
     });
   });
 }
 
 var _undefined = (() => new List(5)[0])();