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This PR introduces a `NativeAssetsManifest.json` next to the `AssetManifest.json` and `FontManifest.json`. This removes the need for embedding the native assets mapping inside the kernel file and will enable decoupling native assets building and bundling from the kernel compilation in flutter tools. This will then allow us to remove dry-run from the build hook protocol. (It also means all isolate groups will have the same native assets. However, since Flutter does not support `Isolate.spawnUri` from kernel files anyways, this is not a regression.) This manifest is parsed eagerly on startup by the engine in a manner similar to how the font manifest is parsed. The manifest contents need to be available in the callback for resolving assets, which does not have access to the engine. Therefore the parsed manifest is `NativeAssetsManager` stored in the `IsolateGroupData`. The engine passes it in on isolate group creation, and the FFI callbacks access it from the isolate group data. Issue: * https://github.com/flutter/flutter/issues/154425 Related PRs: * https://dart-review.googlesource.com/c/sdk/+/388161 Follow up work: * This PR does not yet remove the engine callbacks registered via the dart_api that rely on kernel embedding. If we were to do that in this PR, it would require a manual roll of the engine into flutter/flutter with the PR that switches flutter_tools to emit the native assets manifest instead of embedding in kernel, and a manual roll into g3 to switch emitting a manifest instead of embedding in kernel. A TODO is left in the code for those callbacks to be removed. ## Testing Most of this PR cannot be tested in isolation. The code in this PR is heavily exercised in the follow up flutter_tools PR which creates the `NativeAssetsManifest.json` and removes the embedding of native assets in kernel files. * This PR adds a unit test for parsing the JSON manifest. [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style