Use stream matchers to unflake the mac OS watcher tests.

pkg/watcher/pubspec.yaml

Index: pkg/watcher/pubspec.yaml
diff --git a/pkg/watcher/pubspec.yaml b/pkg/watcher/pubspec.yaml
index c77dddfd2f37c6e82d6a4918250624cb0da9e96c..bf00d9e31fdc54a1835ebb05ae60db94b008e2c4 100644
--- a/pkg/watcher/pubspec.yaml
+++ b/pkg/watcher/pubspec.yaml
@@ -9,7 +9,7 @@ dependencies:
   path: ">=0.9.0 <2.0.0"
   stack_trace: ">=0.9.1 <0.10.0"
 dev_dependencies:
-  scheduled_test: ">=0.9.0 <0.10.0"
-  unittest: ">=0.9.0 <0.10.0"
+  scheduled_test: ">=0.9.3-dev <0.10.0"
+  unittest: ">=0.9.2 <0.10.0"
 environment:
   sdk: ">=0.8.10+6 <2.0.0"