update flutter sdk version constraint

Author: hnvn

example/ios/Flutter/flutter_export_environment.sh

@@ -2,10 +2,9 @@
 # This is a generated file; do not edit or check into version control.
 export "FLUTTER_ROOT=/Users/hunghd/Documents/Workspace/flutter"
 export "FLUTTER_APPLICATION_PATH=/Users/hunghd/Documents/Workspace/flutter_packages/flutter_downloader/example"
-export "FLUTTER_TARGET=/Users/hunghd/Documents/Workspace/flutter_packages/flutter_downloader/example/lib/main.dart"
+export "FLUTTER_TARGET=lib/main.dart"
 export "FLUTTER_BUILD_DIR=build"
 export "SYMROOT=${SOURCE_ROOT}/../build/ios"
 export "FLUTTER_FRAMEWORK_DIR=/Users/hunghd/Documents/Workspace/flutter/bin/cache/artifacts/engine/ios"
 export "FLUTTER_BUILD_NAME=1.0.0"
 export "FLUTTER_BUILD_NUMBER=1"
-export "TRACK_WIDGET_CREATION=true"

pubspec.yaml

@@ -9,6 +9,7 @@ maintainer: Hung Duy Ha (@hnvn)
 
 environment:
   sdk: '>=1.20.1 <3.0.0'
+  flutter: ^1.12.13
 
 dependencies:
   flutter: