Add --target support for Windows and Linux (#34660)

- Adds desktop projects to ApplicationPackageStore
- Plumbs target overrides through the desktop builds

packages/flutter_tools/bin/tool_backend.dart

@@ -13,7 +13,7 @@ Future<void> main(List<String> arguments) async {
   final String projectDirectory = Platform.environment['PROJECT_DIR'];
   final bool verbose = Platform.environment['VERBOSE_SCRIPT_LOGGING'] != null;
   final bool trackWidgetCreation = Platform.environment['TRACK_WIDGET_CREATION'] != null;
-  final String flutterTarget = Platform.environment['FLUTTER_TARGET'] ?? 'lib/main.dart';
+  final String flutterTarget = Platform.environment['FLUTTER_TARGET'] ?? path.join('lib', 'main.dart');
   final String flutterEngine = Platform.environment['FLUTTER_ENGINE'];
   final String localEngine = Platform.environment['LOCAL_ENGINE'];
   final String flutterRoot = Platform.environment['FLUTTER_ROOT'];

packages/flutter_tools/lib/src/bundle.dart

@@ -18,7 +18,7 @@ import 'devfs.dart';
 import 'globals.dart';
 import 'project.dart';
 
-const String defaultMainPath = 'lib/main.dart';
+String get defaultMainPath => fs.path.join('lib', 'main.dart');
 const String defaultAssetBasePath = '.';
 const String defaultManifestPath = 'pubspec.yaml';
 String get defaultDepfilePath => fs.path.join(getBuildDirectory(), 'snapshot_blob.bin.d');
@@ -55,7 +55,7 @@ class BundleBuilder {
   Future<void> build({
     TargetPlatform platform,
     BuildMode buildMode,
-    String mainPath = defaultMainPath,
+    String mainPath,
     String manifestPath = defaultManifestPath,
     String applicationKernelFilePath,
     String depfilePath,
@@ -70,6 +70,7 @@ class BundleBuilder {
     List<String> fileSystemRoots,
     String fileSystemScheme,
   }) async {
+    mainPath ??= defaultMainPath;
     depfilePath ??= defaultDepfilePath;
     assetDirPath ??= getAssetBuildDirectory();
     packagesPath ??= fs.path.absolute(PackageMap.globalPackagesPath);

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

@@ -16,6 +16,7 @@ import 'build.dart';
 /// A command to build a linux desktop target through a build shell script.
 class BuildLinuxCommand extends BuildSubCommand {
   BuildLinuxCommand() {
+    usesTargetOption();
     argParser.addFlag('debug',
       negatable: false,
       help: 'Build a debug version of your app.',
@@ -59,7 +60,7 @@ class BuildLinuxCommand extends BuildSubCommand {
     if (!flutterProject.linux.existsSync()) {
       throwToolExit('No Linux desktop project configured.');
     }
-    await buildLinux(flutterProject.linux, buildInfo);
+    await buildLinux(flutterProject.linux, buildInfo, target: targetFile);
     return null;
   }
 }

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

@@ -16,6 +16,7 @@ import 'build.dart';
 /// A command to build a windows desktop target through a build shell script.
 class BuildWindowsCommand extends BuildSubCommand {
   BuildWindowsCommand() {
+    usesTargetOption();
     argParser.addFlag('debug',
       negatable: false,
       help: 'Build a debug version of your app.',
@@ -59,7 +60,7 @@ class BuildWindowsCommand extends BuildSubCommand {
     if (!flutterProject.windows.existsSync()) {
       throwToolExit('No Windows desktop project configured.');
     }
-    await buildWindows(flutterProject.windows, buildInfo);
+    await buildWindows(flutterProject.windows, buildInfo, target: targetFile);
     return null;
   }
 }

packages/flutter_tools/lib/src/linux/linux_device.dart

@@ -81,7 +81,11 @@ class LinuxDevice extends Device {
   }) async {
     _lastBuiltMode = debuggingOptions.buildInfo.mode;
     if (!prebuiltApplication) {
-      await buildLinux(FlutterProject.current().linux, debuggingOptions.buildInfo);
+      await buildLinux(
+        FlutterProject.current().linux,
+        debuggingOptions.buildInfo,
+        target: mainPath,
+      );
     }
     await stopApp(package);
     final Process process = await processManager.start(<String>[

packages/flutter_tools/lib/src/windows/build_windows.dart

@@ -18,13 +18,15 @@ import 'msbuild_utils.dart';
 import 'visual_studio.dart';
 
 /// Builds the Windows project using msbuild.
-Future<void> buildWindows(WindowsProject windowsProject, BuildInfo buildInfo, {String target = 'lib/main.dart'}) async {
+Future<void> buildWindows(WindowsProject windowsProject, BuildInfo buildInfo, {String target}) async {
   final Map<String, String> environment = <String, String>{
     'FLUTTER_ROOT': Cache.flutterRoot,
-    'FLUTTER_TARGET': target,
     'PROJECT_DIR': windowsProject.project.directory.path,
     'TRACK_WIDGET_CREATION': (buildInfo?.trackWidgetCreation == true).toString(),
   };
+  if (target != null) {
+    environment['FLUTTER_TARGET'] = target;
+  }
   if (artifacts is LocalEngineArtifacts) {
     final LocalEngineArtifacts localEngineArtifacts = artifacts;
     final String engineOutPath = localEngineArtifacts.engineOutPath;

packages/flutter_tools/lib/src/windows/windows_device.dart

@@ -82,7 +82,11 @@ class WindowsDevice extends Device {
     bool ipv6 = false,
   }) async {
     if (!prebuiltApplication) {
-      await buildWindows(FlutterProject.current().windows, debuggingOptions.buildInfo);
+      await buildWindows(
+        FlutterProject.current().windows,
+        debuggingOptions.buildInfo,
+        target: mainPath,
+      );
     }
     await stopApp(package);
     final Process process = await processManager.start(<String>[

packages/flutter_tools/test/commands/attach_test.dart

@@ -34,7 +34,7 @@ void main() {
     setUp(() {
       Cache.disableLocking();
       testFileSystem.directory('lib').createSync();
-      testFileSystem.file('lib/main.dart').createSync();
+      testFileSystem.file(testFileSystem.path.join('lib', 'main.dart')).createSync();
     });
 
     group('with one device and no specified target file', () {
@@ -246,7 +246,7 @@ void main() {
         ..createSync();
 
       // Delete the main.dart file to be sure that attach works without it.
-      fs.file('lib/main.dart').deleteSync();
+      fs.file(fs.path.join('lib', 'main.dart')).deleteSync();
 
       final AttachCommand command = AttachCommand(hotRunnerFactory: mockHotRunnerFactory);
       await createTestCommandRunner(command).run(<String>['attach', '-t', foo.path, '-v']);

packages/flutter_tools/test/commands/build_linux_test.dart

@@ -63,6 +63,7 @@ void main() {
     fs.file('linux/build.sh').createSync(recursive: true);
     fs.file('pubspec.yaml').createSync();
     fs.file('.packages').createSync();
+    fs.file(fs.path.join('lib', 'main.dart')).createSync(recursive: true);
 
     expect(createTestCommandRunner(command).run(
       const <String>['build', 'linux']
@@ -78,6 +79,7 @@ void main() {
     fs.file('linux/build.sh').createSync(recursive: true);
     fs.file('pubspec.yaml').createSync();
     fs.file('.packages').createSync();
+    fs.file(fs.path.join('lib', 'main.dart')).createSync(recursive: true);
 
     when(mockProcessManager.start(<String>[
       'make',

packages/flutter_tools/test/commands/build_windows_test.dart

@@ -87,6 +87,7 @@ void main() {
     when(mockVisualStudio.vcvarsPath).thenReturn(vcvarsPath);
     fs.file('pubspec.yaml').createSync();
     fs.file('.packages').createSync();
+    fs.file(fs.path.join('lib', 'main.dart')).createSync(recursive: true);
 
     expect(createTestCommandRunner(command).run(
       const <String>['build', 'windows']
@@ -104,6 +105,7 @@ void main() {
     when(mockVisualStudio.vcvarsPath).thenReturn(vcvarsPath);
     fs.file('pubspec.yaml').createSync();
     fs.file('.packages').createSync();
+    fs.file(fs.path.join('lib', 'main.dart')).createSync(recursive: true);
 
     when(mockProcessManager.start(<String>[
       r'C:\packages\flutter_tools\bin\vs_build.bat',