Put the testing lib in the flutter namespace. (#8661)

https://dart.googlesource.com/external/github.com/flutter/engine/+/2e4f0a4a72d1b051ff0f114289594975beeca6ba
diff --git a/commits.json b/commits.json
index 6ce60cb..4b20dd4 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"1239df96aada4f995efd2627b032215e7232692f",
+  "external/github.com/flutter/engine":"2e4f0a4a72d1b051ff0f114289594975beeca6ba",
   "external/github.com/flutter/flutter":"0545c63b9baca0d40c2126dba6366fad2b16d3c1",
   "sdk":"76f177a49257056adc92d715ad3a0cc0eae16633"
 }
\ No newline at end of file