[Impeller] Remove OOB access & unused nodiscard statements in tests (#36404)

https://dart.googlesource.com/external/github.com/flutter/engine/+/315d0c2051c5d0a91e0b402c598e1ff36461aa89
diff --git a/commits.json b/commits.json
index 4308d03..d809345 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
-  "external/github.com/flutter/engine":"5e504af1dc14f7945b19aed13314ff8ad2704c16",
+  "external/github.com/flutter/engine":"315d0c2051c5d0a91e0b402c598e1ff36461aa89",
   "external/github.com/flutter/flutter":"9382378f13d2bcddfbfd9a5fe3fa64ad140b517d",
   "sdk":"cfddeee68e0cf7451a2eee1a9fbbff8a8d9c0b92"
 }
\ No newline at end of file