Move minitest.dart into the expect package.

pkg/dev_compiler/test/codegen/lib/html/hidden_dom_2_test.dart

 import 'dart:html';
 
-import 'package:minitest/minitest.dart';
+import 'package:expect/minitest.dart';
 
 // Test that the dart:html API does not leak native jsdom methods:
 //   appendChild operation.
 
 main() {
   test('test1', () {
     document.body.children.add(new Element.html(r'''
 <div id='div1'>
 Hello World!
 </div>'''));
     Element e = document.query('#div1');
     Element e2 = new Element.html(r"<div id='xx'>XX</div>");
     expect(e, isNotNull);
 
     expect(() { confuse(e).appendChild(e2); }, throwsNoSuchMethodError);
 
   });
 }
 
 class Decoy {
   void appendChild(x) { throw 'dead code'; }
 }
 
 confuse(x) => opaqueTrue() ? x : (opaqueTrue() ? new Object() : new Decoy());
 
 /** Returns `true`, but in a way that confuses the compiler. */
 opaqueTrue() => true;  // Expand as needed.