Return devtools serve response instead of printing (#62702)

packages/flutter_tools/doc/daemon.md

@@ -256,7 +256,7 @@ The returned `params` will contain:
 
 #### devtools.serve
 
-The `serve()` command starts a DevTools server if one isn't already running and prints out the host and port of the server.
+The `serve()` command starts a DevTools server if one isn't already running and returns the host and port of the server.
 
 ## 'flutter run --machine' and 'flutter attach --machine'
 

packages/flutter_tools/lib/src/commands/daemon.dart

@@ -896,14 +896,14 @@ class DevToolsDomain extends Domain {
 
   DevtoolsLauncher _devtoolsLauncher;
 
-  Future<void> serve([ Map<String, dynamic> args ]) async {
+  Future<Map<String, dynamic>> serve([ Map<String, dynamic> args ]) async {
     _devtoolsLauncher ??= DevtoolsLauncher.instance;
     final HttpServer server = await _devtoolsLauncher.serve();
 
-    sendEvent('devtools.serve', <String, dynamic>{
+    return<String, dynamic>{
       'host': server.address.host,
       'port': server.port,
-    });
+    };
   }
 
   @override

packages/flutter_tools/test/commands.shard/hermetic/daemon_test.dart

@@ -321,10 +321,10 @@ void main() {
       when(mockDevToolsLauncher.serve()).thenAnswer((_) async => mockDevToolsServer);
 
       commands.add(<String, dynamic>{'id': 0, 'method': 'devtools.serve'});
-      final Map<String, dynamic> response = await responses.stream.firstWhere(_isDevToolsEvent);
-      expect(response['params'], isNotEmpty);
-      expect(response['params']['host'], equals('127.0.0.1'));
-      expect(response['params']['port'], equals(1234));
+      final Map<String, dynamic> response = await responses.stream.firstWhere((Map<String, dynamic> response) => response['id'] == 0);
+      expect(response['result'], isNotEmpty);
+      expect(response['result']['host'], equals('127.0.0.1'));
+      expect(response['result']['port'], equals(1234));
       await responses.close();
       await commands.close();
     }, overrides: <Type, Generator>{
@@ -466,8 +466,6 @@ bool _notEvent(Map<String, dynamic> map) => map['event'] == null;
 
 bool _isConnectedEvent(Map<String, dynamic> map) => map['event'] == 'daemon.connected';
 
-bool _isDevToolsEvent(Map<String, dynamic> map) => map['event'] == 'devtools.serve';
-
 class MockFuchsiaWorkflow extends FuchsiaWorkflow {
   MockFuchsiaWorkflow({ this.canListDevices = true });