Print correct hostname when web server is launched (#42531)

packages/flutter_tools/lib/src/build_runner/web_fs.dart

@@ -209,6 +209,7 @@ class WebFs {
     };
 
     // Initialize the dwds server.
+    final String effectiveHostname = hostname ?? _kHostName;
     final int hostPort = port == null ? await os.findFreePort() : int.tryParse(port);
 
     final Pipeline pipeline = const Pipeline().addMiddleware((Handler innerHandler) {
@@ -279,10 +280,10 @@ class WebFs {
         final BuildRunnerAssetHandler assetHandler = BuildRunnerAssetHandler(
           daemonAssetPort,
           kBuildTargetName,
-          hostname ?? _kHostName,
+          effectiveHostname,
           hostPort);
         dwds = await dwdsFactory(
-          hostname: hostname ?? _kHostName,
+          hostname: effectiveHostname,
           assetHandler: assetHandler,
           buildResults: filteredBuildResults,
           chromeConnection: () async {
@@ -309,13 +310,13 @@ class WebFs {
     Cascade cascade = Cascade();
     cascade = cascade.add(handler);
     cascade = cascade.add(assetServer.handle);
-    final HttpServer server = await httpMultiServerFactory(hostname ?? _kHostName, hostPort);
+    final HttpServer server = await httpMultiServerFactory(effectiveHostname, hostPort);
     shelf_io.serveRequests(server, cascade.handler);
     final WebFs webFS = WebFs(
       client,
       server,
       dwds,
-      'http://$_kHostName:$hostPort/',
+      'http://$effectiveHostname:$hostPort/',
       assetServer,
       buildInfo.isDebug,
       flutterProject,

packages/flutter_tools/test/general.shard/web/web_fs_test.dart

@@ -167,7 +167,7 @@ void main() {
 
   test('Uses provided port number and hostname.', () => testbed.run(() async {
     final FlutterProject flutterProject = FlutterProject.current();
-    await WebFs.start(
+    final WebFs webFs = await WebFs.start(
       skipDwds: false,
       target: fs.path.join('lib', 'main.dart'),
       buildInfo: BuildInfo.debug,
@@ -177,6 +177,7 @@ void main() {
       port: '1234',
     );
 
+    expect(webFs.uri, contains('foo:1234'));
     expect(lastPort, 1234);
     expect(lastAddress, contains('foo'));
   }));