Remove usage of dart:json.

samples/third_party/dromaeo/tests/dom-query-html.dart

 library dromaeo;
 import 'dart:async';
 import 'dart:html';
 import '../common/common.dart';
-import 'dart:json' as json;
+import "dart:convert";
 import 'dart:math' as Math;
 part 'Common.dart';
 part 'RunnerSuite.dart';
 
 void main() {
   final int num = 40;
 
   // Try to force real results.
   var ret;
 
@@ skipping 81 matching lines @@
       for (int i = 0; i < num * 20; i++) {
         ret = document.getElementsByName('test').length == 0;
         ret = document.getElementsByName('test').length == 0;
         ret = document.getElementsByName('test').length == 0;
         ret = document.getElementsByName('test').length == 0;
         ret = document.getElementsByName('test').length == 0;
       }
     })
     .end();
 }