Move the serial_port native code

third_party/serial_port/lib/serial_port.dart

 // Copyright (c) 2014-2015, Nicolas François
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 // http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
 library serial_port;
 
 import 'dart:async';
 import 'dart:isolate';
 import 'dart:io';
-import 'dart-ext:src/serial_port';
+import 'dart-ext:serial_port';
 import 'dart:typed_data';
 
 part 'src/serial_port_impl.dart';
 part 'src/helpers.dart';