pkg/third_party/html5lib: lots of cleanup

pkg/third_party/html5lib/test/dom_compat_test.dart

 library dom_compat_test;
 
-import 'dart:async';
 import 'dart:io';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/compact_vm_config.dart';
 import 'package:html5lib/dom.dart';
 
 part 'dom_compat_test_definitions.dart';
 
 main() {
   useCompactVMConfiguration();
 
   registerDomCompatTests();
 
   test('content_shell', () {
     _runDrt('test/browser/browser_test.html');
   });
 }
 
 void _runDrt(String htmlFile) {
   final allPassedRegExp = new RegExp('All \\d+ tests passed');
 
   final future = Process.run('content_shell', ['--dump-render-tree', htmlFile])
     .then((ProcessResult pr) {
       expect(pr.exitCode, 0);
       expect(pr.stdout, matches(allPassedRegExp), reason: pr.stdout);
     });
 
   expect(future, completion(isNull));
 }