Fix duplicate work in native assets release builds (#158980)

In release builds linking of native assets is enabled. The build step is
only a temprary step, it's output is given to the link step which then
returns all final assets (effectively a map-reduce system). Assets that
aren't sent to a specific linker could be conceptually viewed as sent to
a linker that will emit it's input as-is.

=> The code currently took output of build & link step and therefore
accumulated assets that aren't explicitly sent to a linker twice.

=> This led to performing work twice for those (e.g. copying them twice)

This PR changes this such that if linking mode is enabled, we only rely
on the output of the link phase.
That in return means many tests that mock the native asset builds need
to be updated to mock the output of the link phase.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/256359194e2b6efbf0cd7148661125bd1a68f5d5

DEPS

diff --git a/DEPS b/DEPS
index 56eb3e3..7d7fc55 100644
--- a/DEPS
+++ b/DEPS
@@ -262,7 +262,7 @@
 
   'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '25a9b7f5d2d5c15c47cbd0d8f1f3582565aa8b63',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'eaa99f28b1cebf39be5fe92633d79af0b64fe329',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '256359194e2b6efbf0cd7148661125bd1a68f5d5',
 
   '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 bf2d409..a6354a5 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"cfbd06c0592a402961186e8d9d5374cedceee05a",
   "engine/src/flutter/third_party/dart":"25a9b7f5d2d5c15c47cbd0d8f1f3582565aa8b63",
-  "flutter":"eaa99f28b1cebf39be5fe92633d79af0b64fe329"
+  "flutter":"256359194e2b6efbf0cd7148661125bd1a68f5d5"
 }
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