Introduce a GN rule that (explicitly) generates a dart test wrapper (flutter/engine#55475)

Closes https://github.com/flutter/flutter/issues/155769.

This is a variant of the approach in https://github.com/flutter/engine/pull/52241, based on feedback from @jakemac53 and @jonahwilliams (who originally sped up `dart test` significantly by using `frontend_server` under the scenes: https://github.com/dart-lang/test/pull/1399), in short:

```gn
# tools/engine_tool/BUILD.gn

import("//flutter/build/dart/internal/gen_dartcli_call.gni")

gen_dartcli_call("tests") {
  args = [ "test" ]
  cwd = "//flutter/tools/engine_tool"
}
```

This stanza, when built (`ninja -C ../out/host_debug flutter/tools/engine_tool:tests`) generates the following file:

```sh
# ../out/host_debug/gen/flutter/tools/engine_tool/tests

set -e

# Needed because if it is set, cd may print the path it changed to.
unset CDPATH

# Store the current working directory.
prev_cwd=$(pwd)

# Set a trap to restore the working directory.
trap 'cd "$prev_cwd"' EXIT

CD_PATH="/Users/matanl/Developer/engine/src/flutter/tools/engine_tool"
if [ -n "$CD_PATH" ]; then
  cd "$CD_PATH"
fi

/Users/matanl/Developer/engine/src/flutter/prebuilts/macos-arm64/dart-sdk/bin/dart "test"
```

In turn, when executed (`../out/host_debug/gen/flutter/tools/engine_tool/tests`) it just runs `dart test`:

```sh
flutter % ../out/host_debug/gen/flutter/tools/engine_tool/tests
Building package executable... 
Built test:test.
00:00 +0: test/test_command_test.dart: test command executes test                                                                                                                                                                                                          
00:00 +3: test/run_command_test.dart: run command invokes flutter run
...
00:00 +117: All tests passed!
```

There is no actual compilation performed by the tool (that is handled implicitly by `dart test`), and as a result neither a `depfile` is needed, nor generating a pre-compiled artifact like a snapshot or AOT elf/assembly. 

---

This work is incomplete, that is, we'd want to properly tag the executable so `et` can find it, and create a wrapper template (i.e. `dart_test`) that tightens things up a bit, but I wanted to show the work at this intermediate step to get feedback before moving forward.

/cc @jonahwilliams, @jtmcdole as well.

engine/src/flutter/BUILD.gn

@@ -129,8 +129,11 @@ group("flutter") {
 
   if (build_engine_artifacts) {
     public_deps += [
+      "//flutter/build/dart/test:gen_dartcli_call",
+      "//flutter/build/dart/test:gen_executable_call",
       "//flutter/shell/testing",
       "//flutter/tools/const_finder",
+      "//flutter/tools/engine_tool:tests",
       "//flutter/tools/font_subset",
     ]
   }

engine/src/flutter/build/dart/internal/gen_dartcli_call.gni

@@ -0,0 +1,46 @@
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//flutter/build/dart/internal/gen_executable_call.gni")
+import("//flutter/common/config.gni")
+
+# Generates an executable that runs `dart` with a command and set of arguments.
+#
+# Parameters:
+#   args (optional):
+#     Arguments to pass to the Dart CLI.
+#
+#   cwd (optional):
+#     The working directory for the command.
+#
+#   output (optional):
+#     Overrides the full output path.
+#     Defaults to $root_out_dir/gen/$target_path/$target_name; for example
+#     //flutter/foo/bar emits a binary at out/{variant}/gen/flutter/foo/bar.
+template("gen_dartcli_call") {
+  # Build a reference to the Dart CLI (depending on prebuilt or source).
+  ext = ""
+  if (is_win) {
+    ext = ".exe"
+  }
+  dart = rebase_path("$host_prebuilt_dart_sdk/bin/dart$ext")
+
+  # Add default arguments to the Dart CLI.
+  dart_args = []
+  if (defined(invoker.args)) {
+    dart_args += invoker.args
+  }
+  dart_args += [ "--suppress-analytics" ]
+
+  # Actually generate the shell script.
+  gen_executable_call(target_name) {
+    command = rebase_path(dart)
+    args = dart_args
+    forward_variables_from(invoker,
+                           [
+                             "cwd",
+                             "output",
+                           ])
+  }
+}

engine/src/flutter/build/dart/internal/gen_executable_call.gni

@@ -0,0 +1,62 @@
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Generates an executable that runs a command and set of arguments.
+#
+# Parameters:
+#   command (required):
+#     The command to run, which is typically an absolute path to an executable.
+#
+#   args (optional):
+#     Arguments to pass to the command.
+#
+#   cwd (optional):
+#     The working directory for the command.
+#
+#   output (optional):
+#     Overrides the full output path.
+#     Defaults to $root_out_dir/gen/$target_path/$target_name; for example
+#     //flutter/foo/bar emits a binary at out/{variant}/gen/flutter/foo/bar.
+template("gen_executable_call") {
+  assert(defined(invoker.command), "Must specify 'command'")
+
+  # The command to run.
+  command = invoker.command
+
+  # The output path.
+  output = "$root_gen_dir/$target_name"
+  if (defined(invoker.output)) {
+    output = invoker.output
+  } else {
+    # Construct the output path.
+    output = get_label_info(target_name, "target_gen_dir")
+  }
+
+  # Build the command line arguments.
+  call_args = [
+    "--output",
+    rebase_path(output),
+    "--command",
+    command,
+  ]
+  if (defined(invoker.cwd)) {
+    call_args += [
+      "--cwd",
+      rebase_path(invoker.cwd),
+    ]
+  }
+  if (defined(invoker.args)) {
+    call_args += [
+      "--",
+      string_join(" ", invoker.args),
+    ]
+  }
+
+  # Run build_cmd.py to generate the file and make it executable.
+  action(target_name) {
+    script = "//flutter/build/dart/internal/gen_executable_call.py"
+    outputs = [ output ]
+    args = call_args
+  }
+}

engine/src/flutter/build/dart/internal/gen_executable_call.py

@@ -0,0 +1,55 @@
+#!/usr/bin/env python3
+#
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Generates a shell or batch script to run a command."""
+
+import argparse
+import os
+import string
+
+
+def main():
+  parser = argparse.ArgumentParser(description=__doc__)
+  parser.add_argument('--output', required=True, help='Output file')
+  parser.add_argument('--command', required=True, help='Command to run')
+  parser.add_argument('--cwd', required=False, help='Working directory')
+  parser.add_argument('rest', nargs='*', help='Arguments to pass to the command')
+
+  # Rest of the arguments are passed to the command.
+  args = parser.parse_args()
+
+  out_path = os.path.dirname(args.output)
+  if not os.path.exists(out_path):
+    os.makedirs(out_path)
+
+  script = string.Template(
+      '''#!/bin/sh
+
+set -e
+
+# Set a trap to restore the working directory.
+trap "popd > /dev/null" EXIT
+pushd "$cwd" > /dev/null
+
+$command $args
+'''
+  )
+
+  params = {
+      'command': args.command,
+      'args': ' '.join(args.rest),
+      'cwd': args.cwd if args.cwd else '',
+  }
+
+  with open(args.output, 'w') as f:
+    f.write(script.substitute(params))
+
+  # Make the script executable.
+  os.chmod(args.output, 0o755)
+
+
+if __name__ == '__main__':
+  main()

engine/src/flutter/build/dart/test/BUILD.gn

@@ -0,0 +1,36 @@
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//flutter/build/dart/internal/gen_dartcli_call.gni")
+import("//flutter/build/dart/internal/gen_executable_call.gni")
+
+# Manual test targets that can be used to iterate on individual rules.
+
+# ninja -C ../out/host_debug flutter/build/dart/test:gen_executable_call
+# ../out/host_debug/gen/flutter/build/dart/test/gen_executable_call
+#
+# Expected output: "Hello, World!"
+gen_executable_call("gen_executable_call") {
+  command = "echo"
+  args = [ "Hello, World!" ]
+}
+
+# ninja -C ../out/host_debug flutter/build/dart/test:gen_executable_call_with_cwd
+# ../out/host_debug/gen/flutter/build/dart/test/gen_executable_call_with_cwd
+#
+# Expected output: {varies}/flutter/build/dart/test
+gen_executable_call("gen_executable_call_with_cwd") {
+  command = "echo \$PWD"
+  cwd = "//flutter/build/dart/test"
+}
+
+# ninja -C ../out/host_debug flutter/build/dart/test:gen_dartcli_call
+# ../out/host_debug/gen/flutter/build/dart/test/gen_dartcli_call
+#
+# Expected output: Dart SDK version: 3.6.0-273.0.dev (dev)
+#
+# ... or something like ^, obviously it will constantly change as the SDK rolls.
+gen_dartcli_call("gen_dartcli_call") {
+  args = [ "--version" ]
+}

engine/src/flutter/tools/engine_tool/BUILD.gn

@@ -0,0 +1,15 @@
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//flutter/build/dart/internal/gen_dartcli_call.gni")
+
+# TODO(matanl): WIP in https://github.com/flutter/flutter/issues/155769.
+#
+# Example usage:
+#   ninja -C ../out/host_debug flutter/tools/engine_tool:tests
+#   ../out/host_debug/gen/flutter/tools/engine_tool/tests
+gen_dartcli_call("tests") {
+  args = [ "test" ]
+  cwd = "//flutter/tools/engine_tool"
+}