Remove usage of dart:json.

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

 library dromaeo;
 import 'dart:async';
 import 'dart:html';
-import 'dart:json' as json;
+import "dart:convert";
 import 'dart:math' as Math;
 part 'Common.dart';
 part 'RunnerSuite.dart';
 
 void main() {
   final int num = 10240;
 
   // Try to force real results.
   var ret;
 
@@ skipping 12 matching lines @@
     .test('setAttribute', () {
         for (int i = 0; i < num; i++)
         a.$dom_setAttribute('id', 'foo');
     })
     .test('element.property = value', () {
       for (int i = 0; i < num; i++)
         a.id = 'foo';
     })
     .end();
 }