[engine_tool] et rejects every Dart test target because their names contain a dot #192422

Steps to reproduce

testing/dart/BUILD.gn generates one compile target per Dart test file, and the file name is part of the target name:

foreach(test, tests) {
  flutter_frontend_server("compile_$test") { ... }
}

so the targets are compile_gpu_test.dart, compile_assets_test.dart, and so on — 55 of them. et refuses every one of them, and the group that depends on them:

$ et build -c host_debug_unopt_arm64 //flutter/testing/dart:compile_gpu_test.dart
ERROR: FormatException: Target name must be a valid identifier.
compile_gpu_test.dart
#0      new Label (package:engine_tool/src/label.dart:37:7)
#1      new TargetPattern (package:engine_tool/src/label.dart:167:19)
#2      TargetPattern.parse (package:engine_tool/src/label.dart:184:12)
#3      BuildCommand.run (package:engine_tool/src/commands/build_command.dart:54:50)

$ et build -c host_debug_unopt_arm64 //flutter/testing/dart:dart
ERROR: FormatException: Target name must be a valid identifier.
compile_assets_test.dart
#0      new Label (package:engine_tool/src/label.dart:37:7)
#1      Label.parse (package:engine_tool/src/label.dart:53:12)
#2      Label.parseGn (package:engine_tool/src/label.dart:70:18)

The group fails too, because it enumerates its dependencies and parses each one.

Expected results

et build and et test accept these labels, since GN accepts them and the engine's own build files produce them. ninja -t targets all lists flutter/testing/dart:compile_gpu_test.dart alongside everything else.

Actual results

Every Dart test target in the engine is unreachable through et. Building one means dropping to ninja directly:

ninja -C engine/src/out/host_debug_unopt_arm64 gen/gpu_test.dart.dill

which works, so this is the label parser rather than the build.

Cause

engine/src/flutter/tools/engine_tool/lib/src/label.dart:

static final RegExp _identifier = RegExp(r'^[a-zA-Z_][a-zA-Z0-9_-]*$');

The same pattern validates both package components and target names, and it has no ., so any target whose name carries a file extension is rejected.

Suggested fix

Allow . in target names. Package components can stay as they are — no directory in the engine tree has a dot in its name, while target names with one are generated by the repository's own BUILD.gn.

Happy to send a PR with a regression test in engine_tool/test/label_test.dart, next to the existing "rejects other invalid characters" case.

Context

Found while running the Dart-side flutter_gpu tests for #192398 on a fresh engine checkout: the documented et path did not work for them, and it took a while to conclude the checkout was fine and the tool was not.