Move minitest.dart into the expect package.

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

 import 'dart:html';
 
-import 'package:minitest/minitest.dart';
+import 'package:expect/minitest.dart';
 
 main() {
   var isCssRuleList =
       predicate((x) => x is List<CssRule>, 'is a List<CssRule>');
 
   test("ClientRectList test", () {
     var sheet = document.styleSheets[0] as CssStyleSheet;
     List<CssRule> rulesList = sheet.cssRules;
     expect(rulesList, isCssRuleList);
   });
 }