Bump minimum Dart version

We can now use Set literals. Linter is happy.

Author: haarts

lib/src/logger.dart

@@ -47,9 +47,9 @@ class Logger {
   static Level level = Level.verbose;
 
   // NOTE: callbacks are soon to be removed
-  static final Set<LogCallback> _logCallbacks = Set();
+  static final Set<LogCallback> _logCallbacks = {};
   // NOTE: callbacks are soon to be removed
-  static final Set<OutputCallback> _outputCallbacks = Set();
+  static final Set<OutputCallback> _outputCallbacks = {};
 
   final LogFilter _filter;
   final LogPrinter _printer;

pubspec.yaml

@@ -4,7 +4,7 @@ version: 0.8.3
 homepage: https://github.com/leisim/logger
 
 environment:
-  sdk: ">=2.1.0 <3.0.0"
+  sdk: ">=2.2.0 <3.0.0"
 
 dependencies:
   io: ^0.3.3