* 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)
