Bumped the Dart SDK requirement to 2.1+.

See: flutter/flutter#26179

Author: artob

CHANGELOG.md

@@ -7,7 +7,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [0.4.0] - 2019-02-14
-### Added
+### Changed
+- Require Dart SDK 2.1+
 
 ## [0.3.5] - 2019-01-26
 ### Changed

pubspec.yaml

@@ -18,7 +18,7 @@ dev_dependencies:
   test: '>=1.3.4 <2.0.0'
 dependency_overrides:
 environment:
-  sdk: '>=2.0.0 <3.0.0'
+  sdk: '>=2.1.0 <3.0.0'
 executables:
 flutter:
   plugin: