Add -t support to flutter run on iOS (#4439)

packages/flutter_tools/lib/src/ios/devices.dart

@@ -180,7 +180,7 @@ class IOSDevice extends Device {
     printTrace('Building ${app.name} for $id');
 
     // Step 1: Install the precompiled/DBC application if necessary.
-    XcodeBuildResult buildResult = await buildXcodeProject(app: app, mode: mode, buildForDevice: true);
+    XcodeBuildResult buildResult = await buildXcodeProject(app: app, mode: mode, target: mainPath, buildForDevice: true);
     if (!buildResult.success) {
       printError('Could not build the precompiled application for the device.');
       return new LaunchResult.failed();