make the flutter run -t target option optional (#5029)

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

@@ -56,7 +56,7 @@ class BuildAotCommand extends BuildSubCommand {
     String typeName = path.basename(tools.getEngineArtifactsDirectory(platform, getBuildMode()).path);
     Status status = logger.startProgress('Building AOT snapshot in ${getModeName(getBuildMode())} mode ($typeName)...');
     String outputPath = await buildAotSnapshot(
-      findMainDartFile(argResults['target']),
+      findMainDartFile(targetFile),
       platform,
       getBuildMode(),
       outputPath: argResults['output-dir'],

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

@@ -208,7 +208,7 @@ class BuildApkCommand extends BuildSubCommand {
       return await buildAndroidWithGradle(
         TargetPlatform.android_arm,
         getBuildMode(),
-        target: argResults['target']
+        target: targetFile
       );
     } else {
       // TODO(devoncarew): This command should take an arg for the output type (arm / x64).
@@ -219,7 +219,7 @@ class BuildApkCommand extends BuildSubCommand {
         manifest: argResults['manifest'],
         resources: argResults['resources'],
         outputFile: argResults['output-file'],
-        target: argResults['target'],
+        target: targetFile,
         flxPath: argResults['flx'],
         aotPath: argResults['aot-path'],
         keystore: (argResults['keystore'] ?? '').isEmpty ? null : new ApkKeystoreInfo(

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

@@ -41,7 +41,7 @@ class BuildFlxCommand extends BuildSubCommand {
     String outputPath = argResults['output-file'];
 
     return await build(
-      mainPath: argResults['target'],
+      mainPath: targetFile,
       manifestPath: argResults['manifest'],
       outputPath: outputPath,
       snapshotPath: argResults['snapshot'],

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

@@ -58,7 +58,7 @@ class BuildIOSCommand extends BuildSubCommand {
     XcodeBuildResult result = await buildXcodeProject(
       app: app,
       mode: getBuildMode(),
-      target: argResults['target'],
+      target: targetFile,
       buildForDevice: !forSimulator,
       codesign: shouldCodesign
     );

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

@@ -160,7 +160,7 @@ class DriveCommand extends RunCommandBase {
   }
 
   String _getTestFile() {
-    String appFile = path.normalize(target);
+    String appFile = path.normalize(targetFile);
 
     // This command extends `flutter start` and therefore CWD == package dir
     String packageDir = getCurrentDirectory();
@@ -266,7 +266,7 @@ void restoreAppStarter() {
 }
 
 Future<int> startApp(DriveCommand command) async {
-  String mainPath = findMainDartFile(command.target);
+  String mainPath = findMainDartFile(command.targetFile);
   if (await fs.type(mainPath) != FileSystemEntityType.FILE) {
     printError('Tried to run $mainPath, but that file does not exist.');
     return 1;
@@ -277,7 +277,7 @@ Future<int> startApp(DriveCommand command) async {
     printTrace('Building an APK.');
     int result = await build_apk.buildApk(
       command.device.platform,
-      target: command.target,
+      target: command.targetFile,
       buildMode: command.getBuildMode()
     );
 

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

@@ -62,7 +62,7 @@ class ListenCommand extends RunCommandBase {
 
       result = await startApp(
         deviceForCommand,
-        target: target,
+        target: targetFile,
         install: firstTime,
         stop: true,
         debuggingOptions: new DebuggingOptions.enabled(getBuildMode()),

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

@@ -40,7 +40,7 @@ class RefreshCommand extends FlutterCommand {
     Directory tempDir = await Directory.systemTemp.createTemp('flutter_tools');
     try {
       String snapshotPath = path.join(tempDir.path, 'snapshot_blob.bin');
-      int result = await createSnapshot(mainPath: argResults['target'], snapshotPath: snapshotPath);
+      int result = await createSnapshot(mainPath: targetFile, snapshotPath: snapshotPath);
 
       if (result != 0) {
         printError('Failed to run the Flutter compiler. Exit code: $result');

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

@@ -33,7 +33,6 @@ abstract class RunCommandBase extends FlutterCommand {
   }
 
   bool get traceStartup => argResults['trace-startup'];
-  String get target => argResults['target'];
   String get route => argResults['route'];
 }
 
@@ -135,7 +134,7 @@ class RunCommand extends RunCommandBase {
     if (argResults['resident']) {
       RunAndStayResident runner = new RunAndStayResident(
         deviceForCommand,
-        target: target,
+        target: targetFile,
         debuggingOptions: options,
         hotMode: argResults['hot']
       );
@@ -150,7 +149,7 @@ class RunCommand extends RunCommandBase {
       // using the `RunAndStayResident` class.
       return startApp(
         deviceForCommand,
-        target: target,
+        target: targetFile,
         stop: argResults['full-restart'],
         install: true,
         debuggingOptions: options,

packages/flutter_tools/lib/src/runner/flutter_command.dart

@@ -53,6 +53,15 @@ abstract class FlutterCommand extends Command {
     _usesTargetOption = true;
   }
 
+  String get targetFile {
+    if (argResults.wasParsed('target'))
+      return argResults['target'];
+    else if (argResults.rest.isNotEmpty)
+      return argResults.rest.first;
+    else
+      return null;
+  }
+
   void usesPubOption() {
     argParser.addFlag('pub',
       defaultsTo: true,
@@ -184,7 +193,7 @@ abstract class FlutterCommand extends Command {
     }
 
     if (_usesTargetOption) {
-      String targetPath = argResults['target'];
+      String targetPath = targetFile;
       if (!FileSystemEntity.isFileSync(targetPath)) {
         printError('Target file "$targetPath" not found.');
         return false;