Remove use of dart:json from third_party.

pkg/third_party/html5lib/test/parser_test.dart

 library parser_test;
 
+import 'dart:convert;
 import 'dart:io';
-import 'dart:json' as json;
 import 'package:path/path.dart' as pathos;
 import 'package:unittest/unittest.dart';
 import 'package:html5lib/dom.dart';
 import 'package:html5lib/parser.dart';
 import 'package:html5lib/parser_console.dart' as parser_console;
 import 'package:html5lib/src/constants.dart';
 import 'package:html5lib/src/inputstream.dart' as inputstream;
 import 'package:html5lib/src/tokenizer.dart';
 import 'package:html5lib/src/utils.dart';
 import 'support.dart';
@@ skipping 92 matching lines @@
         }
 
         index++;
       }
     });
   }
 }
 
 /** Extract the name for the test based on the test input data. */
 _nameFor(String input) {
-  // Using json.parse to unescape other unicode characters
+  // Using JSON.decode to unescape other unicode characters
   var escapeQuote = input
       .replaceAll(new RegExp('\\\\.'), '_')
       .replaceAll(new RegExp('\u0000'), '_')
       .replaceAll('"', '\\"')
       .replaceAll(new RegExp('[\n\r\t]'),'_');
-  return json.parse('"$escapeQuote"');
+  return JSON.decode('"$escapeQuote"');
 }