Move minitest.dart into the expect package.

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

 import 'dart:html';
 
-import 'package:minitest/minitest.dart';
+import 'package:expect/minitest.dart';
 
 import 'utils.dart';
 
 main() {
   test('InnerHTML', () {
     Element element = new Element.tag('div');
     element.id = 'test';
     element.innerHtml = 'Hello World';
     document.body.append(element);
 
@@ skipping 78 matching lines @@
         '<div id="dataDiv" data-my-message="Hello World"></div>',
         treeSanitizer: new NullTreeSanitizer());
     expect(otherDiv.dataset.containsKey('myMessage'), isTrue);
 
     Element anotherDiv = new Element.html(
         '<div id="dataDiv" data-eggs="bacon"></div>',
         treeSanitizer: new NullTreeSanitizer());
     expect(anotherDiv.dataset.containsKey('eggs'), isTrue);
   });
 }