Add debugNetworkImageHttpClientProvider (#32857)

Currently, the fact that NetworkImage uses a static HttpClient
makes it impossible to properly test, as a mock in one test will
be reused in another test. This change fixes that.

https://github.com/flutter/flutter/issues/32374
https://dart.googlesource.com/external/github.com/flutter/flutter/+/2b15b244868e3dc89d1502150f7b4f319d3de7e7
diff --git a/commits.json b/commits.json
index 57977ab..b9aaae8 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"e40768c63a00252062ea1061a6db9e756ccb65e8",
-  "external/github.com/flutter/flutter":"950493ff2046f08c9ff1d7cd4a722f6933b0cde5",
+  "external/github.com/flutter/flutter":"2b15b244868e3dc89d1502150f7b4f319d3de7e7",
   "sdk":"2a85d595ba5541c01d3fd8961c929f9ac4695597"
 }
\ No newline at end of file