chore: adjust test dependency

in order to support dart v1.19
diff --git a/pubspec.yaml b/pubspec.yaml
index 2d88c71..fac02f3 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -6,4 +6,5 @@
 environment:
   sdk: '>=0.8.10+6 <2.0.0'
 dev_dependencies:
-  test: '>=0.12.0 <0.13.0'
+  test: '>=0.12.0 <0.12.23'
+  isolate: '>=1.0.0 <1.1.0'