AssetManifest.json is an asset that is generated by the tool during build that lists available assets and their variants. When loading an image at runtime, the framework reads and parses this file to determine if an asset is available

#113637 aims to replace this file with a new one named AssetManifest.bin, which contains similar data that is encoded using the standard_message_codec package. This change speeds up parsing the manifest at runtime, resulting in faster first asset load--especially for apps with large amounts of assets.

While AssetManifest.json is not officially documented, some users do depend on this file, and removing it outright would break them. For example, Google Fonts reads it to determine whether or not a font can be loaded from the device rather than loading it over HTTP.

As such, the aforementioned PR (#113637) will temporarily leave the generation of AssetManifest.json unchanged, and this should be later removed as a part of a breaking change. On the framework side, Flutter will start using AssetManifest.bin immediately, but in the event that it can't be loaded for whatever reason (e.g. custom tooling within g3 doesn't yet generate the file), the framework will fallback to loading AssetManifest.json when resolving assets.