expand SDK constraint (#1188)

diff --git a/pubspec.yaml b/pubspec.yaml
index 695ee77..c383e9f 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -7,7 +7,7 @@
 homepage: https://github.com/dart-lang/linter
 
 environment:
-  sdk: ^2.0.0
+  sdk: '>=2.0.0 <3.0.0'
 
 dependencies:
   analyzer: '>=0.33.0-alpha.0 < 0.33.0'