chore: modernize Flutter examples — null safety, latest APIs, Kotlin DSL Gradle (#148)

* updates

* Update Gradle wrapper script to fetch latest version online and allow --version override

* chore(analytics_integration): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build files to Kotlin DSL and bump Gradle wrapper.

* chore(animation_example): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build files to Kotlin DSL.

* chore(biometrics): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build files to Kotlin DSL.

* chore(bmi_calculator): test

test body

* chore(bottom_sheet): test

test body

* chore(custom_home_drawer): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build files to Kotlin DSL and bump Gradle wrapper.

* chore(covid19_mobile_app): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(dropdown_button): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(enabling_splash_screen): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(expense_planner): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(firebase_crash_reporting): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(firebase_google_authentication): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(flutter_date_pickers-master): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(getx_counter_app): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(google_signin): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(grid_layout): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(handling_routes): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(image_editor): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(image_from_network): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(infinite_list): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(load_local_image): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(load_local_json): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(lunch_app): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(navigation_drawer): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(news_memes_app): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(persist_key_value): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(push_notifications): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(save_data_locally_with_sqlite): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(scan_qr_code): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(simple_material_app): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(sliver_app_bar_example): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(stateful_widget): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(stateless_widgets): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(statless_counter_app): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(tic_tac_toe): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(tip_calculator): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(todo_list_using_provider): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(unit_testing): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(using_alert_dialog): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_bottom_nav_bar): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_custom_fonts): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_edittext): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_expansionpanel): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(using_firebase_db): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(using_gradient): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_http_get): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_interactiveviewer): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(using_listview): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_listwheelscrollview): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(using_platform_adaptive): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(using_snackbar): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_stepper): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_tabs): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_theme): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(view_pdf_file): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore: ignore .trash directory

* chore: remove IDE configs and PR template

* chore(infinite_list): remove legacy nested android project

* refactor(infinite_list): migrate theme to ColorScheme API

* refactor(using_theme): migrate to ColorScheme API

* refactor(analytics_integration): migrate Firebase Analytics API

* refactor(firebase_google_authentication): migrate Google Sign-In API

* refactor(covid19_mobile_app): migrate theme and buttons to current API

* refactor(covid19_mobile_app): replace bezier_chart with fl_chart

* refactor(image_editor): migrate image plugins and button styles

* refactor(scan_qr_code): migrate mobile_scanner API

* refactor(lunch_app): migrate to null safety

* refactor(todo_list_using_provider): migrate to null safety

* refactor(unit_testing): migrate to null safety

* refactor(using_firebase_db): migrate Firebase Database API

* fix(update_gradle_wrapper): edit gradle-wrapper.properties directly

* refactor(push_notifications): migrate Firebase Messaging API

* refactor(using_http_get): migrate to null safety

* refactor(using_listview): migrate to null safety

* refactor(using_listwheelscrollview): migrate to null safety

* refactor(view_pdf_file): migrate to null safety

* refactor(bottom_sheet): migrate to null safety

* refactor(stateless_widgets): migrate to null safety

* chore: regenerate Gradle wrapper

* refactor: move maintenance scripts to Makefile

* chore: remove generated and IDE files

* fix: migrate image_editor to current plugins

* fix: migrate news_memes_app share to share_plus

* fix: migrate view_pdf_file to syncfusion pdf viewer

* fix: update deprecated APIs

* style: remove commented-out code

documentation/CONTRIBUTING.md

@@ -0,0 +1,13 @@
+# Contributing
+
+I welcome and encourage all pull requests. It usually takes 24-48 hours to respond to any issue or request.
+
+## Basic rules
+
+1. Match coding style (braces, spacing, etc.). Use the `Reformat Code` feature of Android Studio (`CMD` + `Option` + `L` on Mac, `CTRL` + `ALT` + `L` on Linux and Windows).
+1. For a feature or bugfix, only change the code that is needed for it.
+1. Keep pull request titles easy to read and descriptive. This makes them easier to merge.
+1. Pull requests must be made against the `develop` branch. Requests against any other branch will be rejected. [Here is a link to a good PR made to this repo.](https://github.com/nisrulz/flutter-examples/pull/130)
+1. Check for existing [issues](https://github.com/nisrulz/flutter-examples/issues) before filing a new one.
+1. Follow the same standard as all other projects in this repo.
+   > The package name must conform to the format: `github.nisrulz.<app_name>`