Add missing import in test.

tests/html/websql_test.dart

 library WebDBTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:async';
 import 'dart:html';
 
 void fail(message) {
   guardAsync(() {
       expect(false, isTrue, reason: message);
     });
 }
 
 Future<SqlTransaction> createTransaction(Database db) {
   final completer = new Completer<SqlTransaction>();
@@ skipping 93 matching lines @@
           expect(resultSet.rows.length, 1);
           var row = resultSet.rows.item(0);
           expect(row.containsKey(columnName), isTrue);
           expect(row[columnName], 'Some text data');
         });
       }))
       .then(dropTable(tableName))
       .then(expectAsync1((tx) {}));
   });
 }