• Write a tool_executable (name TBD) rule that generates a shell (and later, batch, for Windows) in out/
  - [ ] Write a dart_snapshot rule that generates a [kernel snapshot]¹ and depfile for a given Dart entrypoint
• Write a dart_test rule that generates a tool_executable and dart_snapshot, and uses dart test to run a file

Working on #147013 and wanted to write out some of my thoughts to explain what I think needs to get done, and why it will take a different path the @cbracken's original PR, flutter/engine#52241, and need (likely 1-2 PRs) foundational work as a starting step.

We'd like to get to a world where (something) like the following can be written:

# //flutter/tools/engine_tool/BUILD.gn

import("//flutter/build/dart/rules.gni")

dart_test_suite("tests") {
  sources = ["test/**/*_test.dart"],
}

But let's break that down a bit, and make it more verbose to make it more understandable:

# //flutter/tools/engine_tool/BUILD.gn

import("//flutter/build/dart/rules.gni")

dart_test("build_command_test") {
  script = "test/build_command_test.dart",
}

dart_test("utils_test") {
  script = "test/utils_test.dart",
}

dart_test("worker_pool_test") {
  script = "test/worker_pool_test.dart",
}

Ok, now one step further, what does/would et test //flutter/tools/engine_tool:build_command_test do?

1. Generate a build_command_test.depfile (and as a side-effect, snapshot)

2. Generate a build_command_test.sh file that has contents something like:

   #!/bin/bash
   set -e
   pushd {{WORKING_DIR}}
   {{DART_BINARY}} test test/build_command_test.dart
   popd

3. Runs a ninja build that builds (1) and (2)

4. Runs the executable output by (2), i.e. ../out/host_debug/build_command_test.sh (or whatever)

/cc @jonahwilliams @cbracken @jakemac53 @johnmccutchan