Declare support for Dart 2 (#418)

Author: nex3

pubspec.yaml

@@ -8,7 +8,7 @@ executables:
   dart-sass: sass
 
 environment:
-  sdk: '>=1.25.0-dev.64.0 <2.0.0'
+  sdk: '>=1.25.0-dev.64.0 <3.0.0'
 
 dependencies:
   args: ">=1.4.0 <2.0.0"