PopScope example improvements (#142163)

Attempting to help users understand how to build a confirmation dialog when exiting a route.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/204a8848a609309f1c7f1385ce6ef662a83e0290

DEPS

diff --git a/DEPS b/DEPS
index 6392b86..06b68cc 100644
--- a/DEPS
+++ b/DEPS
@@ -284,7 +284,7 @@
 
   'engine/src/third_party/dart': Var('dart_git') + '/sdk' + '@' + '3b294fdab216daecb537bb45f5921e48a9d62dde',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'e985c29f311c4611ecf3bd76697a00bef5458a27',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '204a8848a609309f1c7f1385ce6ef662a83e0290',
 
   'engine/src/flutter/third_party/rapidjson':
    Var('flutter_git') + '/third_party/rapidjson' + '@' + 'ef3564c5c8824989393b87df25355baf35ff544b',

commits.json

diff --git a/commits.json b/commits.json
index 376437e..abd96de 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"55eefd5bd255cfac769509a7b16601faa5e3d546",
   "engine/src/third_party/dart":"3b294fdab216daecb537bb45f5921e48a9d62dde",
-  "flutter":"e985c29f311c4611ecf3bd76697a00bef5458a27"
+  "flutter":"204a8848a609309f1c7f1385ce6ef662a83e0290"
 }
\ No newline at end of file