normalize -t --target option

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

@@ -159,8 +159,8 @@ class ApkCommand extends FlutterCommand {
         help: 'Output APK file.');
     argParser.addOption('target',
         abbr: 't',
-        defaultsTo: '',
-        help: 'Target app path or filename used to build the FLX.');
+        defaultsTo: flx.defaultMainPath,
+        help: 'Target app path / main entry-point file.');
     argParser.addOption('flx',
         abbr: 'f',
         defaultsTo: '',

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

@@ -17,7 +17,11 @@ class BuildCommand extends FlutterCommand {
     argParser.addFlag('precompiled', negatable: false);
     argParser.addOption('asset-base', defaultsTo: defaultAssetBase);
     argParser.addOption('compiler');
-    argParser.addOption('main', defaultsTo: defaultMainPath);
+    argParser.addOption('target',
+      abbr: 't',
+      defaultsTo: defaultMainPath,
+      help: 'Target app path / main entry-point file.'
+    );
     argParser.addOption('manifest', defaultsTo: defaultManifestPath);
     argParser.addOption('private-key', defaultsTo: defaultPrivateKeyPath);
     argParser.addOption('output-file', abbr: 'o', defaultsTo: defaultFlxOutputPath);

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

@@ -11,6 +11,7 @@ import '../application_package.dart';
 import '../base/common.dart';
 import '../base/context.dart';
 import '../device.dart';
+import '../flx.dart';
 import '../runner/flutter_command.dart';
 import '../toolchain.dart';
 import 'install.dart';
@@ -41,7 +42,8 @@ abstract class StartCommandBase extends FlutterCommand {
         help: 'Start tracing during startup.');
     argParser.addOption('target',
         abbr: 't',
-        help: 'Target app path or filename to start.');
+        defaultsTo: defaultMainPath,
+        help: 'Target app path / main entry-point file.');
     argParser.addOption('route',
         help: 'Which route to load when starting the app.');
   }