Add --target support for Windows and Linux (#34660) - Adds desktop projects to ApplicationPackageStore - Plumbs target overrides through the desktop builds

diff --git a/packages/flutter_tools/lib/src/bundle.dart b/packages/flutter_tools/lib/src/bundle.dart
index 42fd38a..f298114 100644
--- a/packages/flutter_tools/lib/src/bundle.dart
+++ b/packages/flutter_tools/lib/src/bundle.dart

@@ -18,7 +18,7 @@
 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 @@
   Future<void> build({
     TargetPlatform platform,
     BuildMode buildMode,
-    String mainPath = defaultMainPath,
+    String mainPath,
     String manifestPath = defaultManifestPath,
     String applicationKernelFilePath,
     String depfilePath,
@@ -70,6 +70,7 @@
     List<String> fileSystemRoots,
     String fileSystemScheme,
   }) async {
+    mainPath ??= defaultMainPath;
     depfilePath ??= defaultDepfilePath;
     assetDirPath ??= getAssetBuildDirectory();
     packagesPath ??= fs.path.absolute(PackageMap.globalPackagesPath);

diff --git a/packages/flutter_tools/lib/src/commands/build_linux.dart b/packages/flutter_tools/lib/src/commands/build_linux.dart
index c1f15dd..4175b6d 100644
--- a/packages/flutter_tools/lib/src/commands/build_linux.dart
+++ b/packages/flutter_tools/lib/src/commands/build_linux.dart

@@ -16,6 +16,7 @@
 /// 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 @@
     if (!flutterProject.linux.existsSync()) {
       throwToolExit('No Linux desktop project configured.');
     }
-    await buildLinux(flutterProject.linux, buildInfo);
+    await buildLinux(flutterProject.linux, buildInfo, target: targetFile);
     return null;
   }
 }

diff --git a/packages/flutter_tools/lib/src/commands/build_windows.dart b/packages/flutter_tools/lib/src/commands/build_windows.dart
index 2f59aec..884d26f 100644
--- a/packages/flutter_tools/lib/src/commands/build_windows.dart
+++ b/packages/flutter_tools/lib/src/commands/build_windows.dart

@@ -16,6 +16,7 @@
 /// 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 @@
     if (!flutterProject.windows.existsSync()) {
       throwToolExit('No Windows desktop project configured.');
     }
-    await buildWindows(flutterProject.windows, buildInfo);
+    await buildWindows(flutterProject.windows, buildInfo, target: targetFile);
     return null;
   }
 }

diff --git a/packages/flutter_tools/lib/src/linux/linux_device.dart b/packages/flutter_tools/lib/src/linux/linux_device.dart
index 3746aa5..17b8002 100644
--- a/packages/flutter_tools/lib/src/linux/linux_device.dart
+++ b/packages/flutter_tools/lib/src/linux/linux_device.dart

@@ -81,7 +81,11 @@
   }) 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>[

diff --git a/packages/flutter_tools/lib/src/windows/build_windows.dart b/packages/flutter_tools/lib/src/windows/build_windows.dart
index 781b095..9f1a82c 100644
--- a/packages/flutter_tools/lib/src/windows/build_windows.dart
+++ b/packages/flutter_tools/lib/src/windows/build_windows.dart

@@ -18,13 +18,15 @@
 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;

diff --git a/packages/flutter_tools/lib/src/windows/windows_device.dart b/packages/flutter_tools/lib/src/windows/windows_device.dart
index d55df6d..f2e5e10 100644
--- a/packages/flutter_tools/lib/src/windows/windows_device.dart
+++ b/packages/flutter_tools/lib/src/windows/windows_device.dart

@@ -82,7 +82,11 @@
     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>[