Migrate to Null-Safety (#91)
* refactor(nnbd): migrate to null-safety
* refactor(deps): remove merge_map package
* refactor(deps): remove color package
* chore(format): fix format
* feat(cli): support option of --no-sound-null-safety
* refactor(integration): add name of params
* refactor(nndb): add option for pubspec.yaml
* refactor(cli): remove unused var
* refactor(*): copy from dart_style
* refactor(*): refactor
* refactor(plugin): remove color libs
* refactor(plugin): remove color libs
* chore(deps): update deps
* chore(deps): update deps
* chore(analyze): fix analyze points
* chore(deps): add collection
* docs(changelog): update
* chore(*): update deps
* chore(*): bump up 3.0.1
* chore(*): bump up 3.0.1
* chore(deps): update deps
* chore(deps): update deps in sample
