Before #122505 landed, the tool would generate a file named AssetManifest.json which was bundled into the flutter app being built. This file contained information about assets. After #122505, a new file gets generated, AssetManifest.bin, which succeeded AssetManifest.json. However, the tool still produces AssetManifest.json since some customers depend on it to determine what assets are available to an app at runtime. While this was never a publicly documented feature, it was recommended as a workaround to some users. In current times, the AssetManifest provides a supported way to do this, and we want users to use that instead.

We should announce a deprecation/removal of the AssetManifest.json concept, and remove all references to it in code.