[Feature]update dart sdk and add some gitignore

.gitignore

@@ -1,15 +1,133 @@
+# Miscellaneous
+*.class
+*.lock
+*.log
+*.pyc
+*.swp
 .DS_Store
-.dart_tool/
+.atom/
+.buildlog/
+.history
+.svn/
+
+# IntelliJ related
+*.iml
+*.ipr
+*.iws
+.idea/
+
+# Visual Studio Code related
+.classpath
+.project
+.settings/
+.vscode/*
 
+# Flutter repo-specific
+/bin/cache/
+/bin/internal/bootstrap.bat
+/bin/internal/bootstrap.sh
+/bin/mingit/
+/dev/benchmarks/mega_gallery/
+/dev/bots/.recipe_deps
+/dev/bots/android_tools/
+/dev/devicelab/ABresults*.json
+/dev/docs/doc/
+/dev/docs/flutter.docs.zip
+/dev/docs/lib/
+/dev/docs/pubspec.yaml
+/dev/integration_tests/**/xcuserdata
+/dev/integration_tests/**/Pods
+/packages/flutter/coverage/
+version
+analysis_benchmark.json
+
+# packages file containing multi-root paths
+.packages.generated
+
+# Flutter/Dart/Pub related
+**/doc/api/
+.dart_tool/
+.flutter-plugins
+.flutter-plugins-dependencies
+**/generated_plugin_registrant.dart
 .packages
+.pub-preload-cache/
 .pub/
-
 build/
-ios/.generated/
-ios/Flutter/Generated.xcconfig
-ios/Runner/GeneratedPluginRegistrant.*
+flutter_*.png
+linked_*.ds
+unlinked.ds
+unlinked_spec.ds
 
-.idea
+# Android related
+**/android/**/gradle-wrapper.jar
+.gradle/
+**/android/captures/
+**/android/gradlew
+**/android/gradlew.bat
+**/android/local.properties
+**/android/**/GeneratedPluginRegistrant.java
+**/android/key.properties
+*.jks
 
-*.iml
-pubspec.lock
+# iOS/XCode related
+**/ios/**/*.mode1v3
+**/ios/**/*.mode2v3
+**/ios/**/*.moved-aside
+**/ios/**/*.pbxuser
+**/ios/**/*.perspectivev3
+**/ios/**/*sync/
+**/ios/**/.sconsign.dblite
+**/ios/**/.tags*
+**/ios/**/.vagrant/
+**/ios/**/DerivedData/
+**/ios/**/Icon?
+**/ios/**/Pods/
+**/ios/**/.symlinks/
+**/ios/**/profile
+**/ios/**/xcuserdata
+**/ios/.generated/
+**/ios/Flutter/.last_build_id
+**/ios/Flutter/App.framework
+**/ios/Flutter/Flutter.framework
+**/ios/Flutter/Flutter.podspec
+**/ios/Flutter/Generated.xcconfig
+**/ios/Flutter/ephemeral
+**/ios/Flutter/app.flx
+**/ios/Flutter/app.zip
+**/ios/Flutter/flutter_assets/
+**/ios/Flutter/flutter_export_environment.sh
+**/ios/ServiceDefinitions.json
+**/ios/Runner/GeneratedPluginRegistrant.*
+
+# macOS
+**/Flutter/ephemeral/
+**/Pods/
+**/macos/Flutter/GeneratedPluginRegistrant.swift
+**/macos/Flutter/ephemeral
+**/xcuserdata/
+
+# Windows
+**/windows/flutter/generated_plugin_registrant.cc
+**/windows/flutter/generated_plugin_registrant.h
+**/windows/flutter/generated_plugins.cmake
+
+# Linux
+**/linux/flutter/generated_plugin_registrant.cc
+**/linux/flutter/generated_plugin_registrant.h
+**/linux/flutter/generated_plugins.cmake
+
+# Coverage
+coverage/
+
+# Symbols
+app.*.symbols
+
+# Exceptions to above rules.
+!**/ios/**/default.mode1v3
+!**/ios/**/default.mode2v3
+!**/ios/**/default.pbxuser
+!**/ios/**/default.perspectivev3
+!/packages/flutter_tools/test/data/dart_dependencies_test/**/.packages
+!/dev/ci/**/Gemfile.lock
+!.vscode/settings.json

example/pubspec.yaml

@@ -2,7 +2,7 @@ name: example
 description: A new Flutter application.
 
 environment:
-  sdk: ">=2.12.0 <3.0.0"
+  sdk: ">=2.12.0 <4.0.0"
 
 dependencies:
   flutter:

pubspec.yaml

@@ -12,7 +12,7 @@ dev_dependencies:
     sdk: flutter
 
 environment:
-  sdk: ">=2.12.0 <3.0.0"
+  sdk: ">=2.12.0 <4.0.0"
 
 # For information on the generic Dart part of this file, see the
 # following page: https://www.dartlang.org/tools/pub/pubspec