add a hidden flag for backwards compatibility

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

@@ -22,6 +22,8 @@ class BuildCommand extends FlutterCommand {
       defaultsTo: defaultMainPath,
       help: 'Target app path / main entry-point file.'
     );
+    // TODO(devoncarew): Remove this once the xcode project is switched over.
+    argParser.addOption('main', hide: true);
     argParser.addOption('manifest', defaultsTo: defaultManifestPath);
     argParser.addOption('private-key', defaultsTo: defaultPrivateKeyPath);
     argParser.addOption('output-file', abbr: 'o', defaultsTo: defaultFlxOutputPath);
@@ -41,7 +43,7 @@ class BuildCommand extends FlutterCommand {
     return await build(
       toolchain,
       assetBase: argResults['asset-base'],
-      mainPath: argResults['target'],
+      mainPath: argResults.wasParsed('main') ? argResults['main'] : argResults['target'],
       manifestPath: argResults['manifest'],
       outputPath: outputPath,
       snapshotPath: argResults['snapshot'],