Use InternetAddress instead of String (#6922)

packages/flutter_tools/lib/src/test/flutter_platform.dart

@@ -20,7 +20,7 @@ import '../globals.dart';
 import 'coverage_collector.dart';
 
 final String _kSkyShell = Platform.environment['SKY_SHELL'];
-const String _kHost = '127.0.0.1';
+final InternetAddress _kHost = InternetAddress.LOOPBACK_IP_V4;
 const String _kRunnerPath = '/runner';
 const String _kShutdownPath = '/shutdown';
 
@@ -50,8 +50,8 @@ Future<_ServerInfo> _startServer() async {
     else if (!socket.isCompleted && request.uri.path == _kShutdownPath)
       socket.completeError('Failed to start test');
   });
-  return new _ServerInfo(server, 'ws://$_kHost:${server.port}$_kRunnerPath',
-      'ws://$_kHost:${server.port}$_kShutdownPath', socket.future);
+  return new _ServerInfo(server, 'ws://${_kHost.address}:${server.port}$_kRunnerPath',
+      'ws://${_kHost.address}:${server.port}$_kShutdownPath', socket.future);
 }
 
 Future<Process> _startProcess(String mainPath, { String packages, int observatoryPort }) {
@@ -166,7 +166,7 @@ void main() {
         Process processToKill = process;
         process = null;
         CoverageCollector.instance.collectCoverage(
-          host: _kHost,
+          host: _kHost.address,
           port: observatoryPort,
           processToKill: processToKill
         );