Reland "Roll engine to version e3687f70c7ece72000b32ee1b3c02755ba5361ac (#20427)" (#20633)

* Reland "Roll engine to version b148e628ec86b3a9a0382e0bcfae73f0390a8232 (#20427)"

This is a re-land with downgraded `package:flutter_gallery_assets`
version.

* Downgrade package:flutter_gallery_assets to 0.1.4

* Change engine.version to 81baff97c29bb08cbf8453a3f9042c5813f84ad3 (which contains an additional fix)

* Change engine.version to e3687f70c7ece72000b32ee1b3c02755ba5361ac (since mac tarballs are corrupted on earlier commit)
diff --git a/examples/platform_channel_swift/pubspec.yaml b/examples/platform_channel_swift/pubspec.yaml
index cc33a8e..1ae0776 100644
--- a/examples/platform_channel_swift/pubspec.yaml
+++ b/examples/platform_channel_swift/pubspec.yaml
@@ -1,12 +1,16 @@
 name: platform_channel_swift
 
+environment:
+  # The pub client defaults to an <2.0.0 sdk constraint which we need to explicitly overwrite.
+  sdk: ">=2.0.0-dev.68.0 <3.0.0"
+
 dependencies:
   flutter:
     sdk: flutter
 
-  collection: 1.14.6 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  meta: 1.1.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  typed_data: 1.1.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  collection: 1.14.11 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  meta: 1.1.6 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  typed_data: 1.1.6 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   vector_math: 2.0.8 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
 
 dev_dependencies:
@@ -58,7 +62,7 @@
   stream_channel: 1.6.8 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   string_scanner: 1.0.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   term_glyph: 1.0.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
-  test: 0.12.42 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  test: 1.3.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   utf: 0.9.0+5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   vm_service_client: 0.2.6 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   watcher: 0.9.7+10 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
@@ -68,4 +72,4 @@
 flutter:
   uses-material-design: true
 
-# PUBSPEC CHECKSUM: 0264
+# PUBSPEC CHECKSUM: 0c2d