Update 'dump-info' usage for dart2js builds (#156187)

Dart2js updated its CLI to support generating a 'dump-info' json file by passing a "--stage" option. The "dump-info-all" stage performs a full compilation (from the provided dill) and then also generates the dump info file.

Tested via the `flutter-dev` CLI locally. This results in the same output but with the addition of an extra `main.dart.js.info.json` file.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/da71e6e93d78d362b9200cecaa493f4f71c2ebe8

DEPS

diff --git a/DEPS b/DEPS
index dabbca6..16e18bf 100644
--- a/DEPS
+++ b/DEPS
@@ -284,7 +284,7 @@
 
   'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '9b7f5a0af6e6c8033dc8e9bd685d10a85c342555',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '86ce72d46d871a72b705393f711422f13684b7b5',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'da71e6e93d78d362b9200cecaa493f4f71c2ebe8',
 
   'engine/src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',

commits.json

diff --git a/commits.json b/commits.json
index 8a6de41..16657c0 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"14b7e2870a7d6acadd5232058a5f5d127b52fc94",
   "engine/src/flutter/third_party/dart":"9b7f5a0af6e6c8033dc8e9bd685d10a85c342555",
-  "flutter":"86ce72d46d871a72b705393f711422f13684b7b5"
+  "flutter":"da71e6e93d78d362b9200cecaa493f4f71c2ebe8"
 }
\ No newline at end of file