Update the SDK constraint.

This allows us to use ^ constraints.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1646943003 .
diff --git a/pubspec.yaml b/pubspec.yaml
index 46d1c28..5dff060 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -9,6 +9,6 @@
   shelf: ">=0.6.5 <0.8.0"
 dev_dependencies:
   http: ">=0.10.0 <0.12.0"
-  test: ">=0.12.0 <0.13.0"
+  test: "^0.12.0"
 environment:
-  sdk: ">=1.4.0-dev.5.0 <2.0.0"
+  sdk: ">=1.8.0 <2.0.0"