add upper bound on SDK

Change-Id: Ic5518f213c9b8303618a1e0210f7784deac6559b
diff --git a/pkg/kernel/pubspec.yaml b/pkg/kernel/pubspec.yaml
index fa48db1..1753148 100644
--- a/pkg/kernel/pubspec.yaml
+++ b/pkg/kernel/pubspec.yaml
@@ -4,7 +4,7 @@
 description: Dart IR (Intermediate Representation)
 homepage: https://github.com/dart-lang/sdk/tree/master/pkg/kernel
 environment:
-  sdk: ">=1.8.0"
+  sdk: ">=1.8.0 <2.0.0"
 dependencies:
   path: ^1.3.9
   args: '>=0.13.4 <2.0.0'