add daemon command to enumerate supported platforms (#33472)

packages/flutter_tools/doc/daemon.md

@@ -60,6 +60,17 @@ The `version()` command responds with a String with the protocol version.
 
 The `shutdown()` command will terminate the flutter daemon. It is not necessary to call this before shutting down the daemon; it is perfectly acceptable to just kill the daemon process.
 
+### daemon.getSupportedPlatforms
+
+The `getSupportedPlatforms()` command will enumerate all platforms supported by the project located at the provided `projectRoot`. It returns a Map with the key 'platforms' containing a List of strings which describe the set of all possibly supported platforms. Possible values include:
+   - android
+   - ios
+   - linux #experimental
+   - macos #experimental
+   - windows #experimental
+   - fuchsia #experimental
+   - web #experimental
+
 #### Events
 
 #### daemon.connected

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

@@ -248,6 +248,7 @@ class DaemonDomain extends Domain {
   DaemonDomain(Daemon daemon) : super(daemon, 'daemon') {
     registerHandler('version', version);
     registerHandler('shutdown', shutdown);
+    registerHandler('getSupportedPlatforms', getSupportedPlatforms);
 
     sendEvent(
       'daemon.connected',
@@ -300,6 +301,59 @@ class DaemonDomain extends Domain {
   void dispose() {
     _subscription?.cancel();
   }
+
+  /// Enumerates the platforms supported by the provided project.
+  ///
+  /// This does not filter based on the current workflow restrictions, such
+  /// as whether command line tools are installed or whether the host platform
+  /// is correct.
+  Future<Map<String, Object>> getSupportedPlatforms(Map<String, dynamic> args) async {
+    final String projectRoot = _getStringArg(args, 'projectRoot', required: true);
+    final List<String> result = <String>[];
+    try {
+      // TODO(jonahwilliams): replace this with a project metadata check once
+      // that has been implemented.
+      final FlutterProject flutterProject = FlutterProject.fromDirectory(fs.directory(projectRoot));
+      if (flutterProject.linux.existsSync()) {
+        result.add('linux');
+      }
+      if (flutterProject.macos.existsSync()) {
+        result.add('macos');
+      }
+      if (flutterProject.windows.existsSync()) {
+        result.add('windows');
+      }
+      if (flutterProject.ios.existsSync()) {
+        result.add('ios');
+      }
+      if (flutterProject.android.existsSync()) {
+        result.add('android');
+      }
+      if (flutterProject.web.existsSync()) {
+        result.add('web');
+      }
+      if (flutterProject.fuchsia.existsSync()) {
+        result.add('fuchsia');
+      }
+      return <String, Object>{
+        'platforms': result,
+      };
+    } catch (err, stackTrace) {
+      sendEvent('log', <String, dynamic>{
+        'log': 'Failed to parse project metadata',
+        'stackTrace': stackTrace.toString(),
+        'error': true,
+      });
+      // On any sort of failure, fall back to Android and iOS for backwards
+      // comparability.
+      return <String, Object>{
+        'platforms': <String>[
+          'android',
+          'ios',
+        ],
+      };
+    }
+  }
 }
 
 typedef _RunOrAttach = Future<void> Function({