Remove use of dart:json from third_party.

pkg/third_party/html5lib/test/tokenizer_test.dart

 library tokenizer_test;
 
 // Note: mirrors used to match the getattr usage in the original test
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
-import 'dart:json' as json;
 import 'dart:mirrors';
 import 'dart:utf';
 import 'package:path/path.dart' as pathos;
 import 'package:unittest/unittest.dart';
 import 'package:html5lib/src/char_encodings.dart';
 import 'package:html5lib/src/constants.dart' as constants;
 import 'package:html5lib/src/token.dart';
 import 'package:html5lib/src/tokenizer.dart';
 import 'package:html5lib/src/utils.dart';
 import 'support.dart';
@@ skipping 183 matching lines @@
               "\nInput:", testInfo['input'],
               "\nExpected:", expected,
               "\nreceived:", tokens].map((s) => '$s').join('\n');
   var ignoreErrorOrder = testInfo['ignoreErrorOrder'];
   if (ignoreErrorOrder == null) ignoreErrorOrder = false;
 
   expectTokensMatch(expected, received, ignoreErrorOrder, true, errorMsg);
 }
 
 Map unescape(Map testInfo) {
-  // TODO(sigmundch,jmesserly): we currently use json.parse to unescape the
+  // TODO(sigmundch,jmesserly): we currently use JSON.decode to unescape the
   // unicode characters in the string, we should use a decoding that works with
   // any control characters.
-  decode(inp) => inp == '\u0000' ? inp : json.parse('"$inp"');
+  decode(inp) => inp == '\u0000' ? inp : JSON.decode('"$inp"');
 
   testInfo["input"] = decode(testInfo["input"]);
   for (var token in testInfo["output"]) {
     if (token == "ParseError") {
       continue;
     } else {
       token[1] = decode(token[1]);
       if (token.length > 2) {
         for (var pair in token[2]) {
           var key = pair[0];
@@ skipping 17 matching lines @@
     result.write(match.group(2));
   }
   return result.toString();
 }
 
 void main() {
   for (var path in getDataFiles('tokenizer')) {
     if (!path.endsWith('.test')) continue;
 
     var text = new File(path).readAsStringSync();
-    var tests = json.parse(text);
+    var tests = JSON.decode(text);
     var testName = pathos.basenameWithoutExtension(path);
     var testList = tests['tests'];
     if (testList == null) continue;
 
     group(testName, () {
       for (int index = 0; index < testList.length; index++) {
         final testInfo = testList[index];
 
         testInfo.putIfAbsent("initialStates", () => ["Data state"]);
         for (var initialState in testInfo["initialStates"]) {
           test(testInfo["description"], () {
             testInfo["initialState"] = camelCase(initialState);
             runTokenizerTest(testInfo);
           });
         }
       }
     });
   }
 }