* 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)
Reason for revert: The package:flutter_gallery_assets has removed some images which are required for the examples/flutter_gallery, so the gallery build is failing (only discovered after landing, since gallery doesn't seem to get built during github PR presubmit checks)
This CL
* rolls `engine.version` to flutter/engine@b148e628 (which includes dart sdk 2.1.0-dev)
* rolls `goldens.version` to flutter/goldens@6c45fafdf (which includes updates due to skia changes in engine)
* changes `platform.dill` to `platform_strong.dill` in various places due to flutter/engine@a84b210b
* adds explicit `environment: sdk: ">=2.0.0-dev.68 < 3.0.0"` constraints to `pubspec.yaml` and `pubspec.yaml.tmpl` files (since pub defaults to `<2.0.0` if omitted)
* upgrades to newer versions of various 3rd party packages (to ensure transitive dependencies have `<3.0.0` sdk constraint)
* traverse dependencies and dev dependencies separately
* separate dev and reg deps
* update pubspecs
* add checksum string and update transitive dep string. Add additional verify command to check checksum
* cleanup comments
* add missing space, quote, run verify-onlu from bot script
* rerun tool with fixed string
* ensure correct working directory and bump versions
* flutter not flutterRoot
* Unpin package:test and upgrade packages
* Update packages/flutter/test/foundation/stack_trace_test.dart
* Also add packages/flutter_tools/test/data/asset_test/font/.dartignore to ensure that update-packages --force-upgrade does not crash.
