Supporting passing a filename that contains a list of arguments as the last argument

Allows the final argument to be `@filename` where `filename` is a path (relative to `cwd`) to a file that contains arguments to replace it before parsing.

This allows arguments longer than Windows supports to be provided (see Dart-Code/Dart-Code#5473).

Author: DanTup

pkgs/test/CHANGELOG.md

@@ -1,5 +1,8 @@
 ## 1.25.16-wip
 
+* Allow the final argument to be a filename prefixed with `@` that contains a
+  list of arguments as if they had been provided on the command line directly.
+
 ## 1.25.15
 
 * Allow the latest version of `package:shelf_web_socket`.

pkgs/test/test/runner/runner_test.dart

@@ -868,4 +868,41 @@ void main() {
       await test.shouldExit(0);
     }, skip: 'https://github.com/dart-lang/test/issues/1803');
   });
+
+  group('using @file as the final argument', () {
+    test('prints a suitable message if the file does not exist', () async {
+      var test = await runTest(['@mytestfile']);
+      expect(
+          test.stderr,
+          emitsThrough(
+              contains('Failed to read the file "mytestfile" for arguments')));
+      await test.shouldExit(64);
+    });
+
+    test('runs test suites provided in the file', () async {
+      await d.file('test.dart', _success).create();
+      await d.file('mytestargs.txt', 'test.dart').create();
+      var test = await runTest(['@mytestargs.txt']);
+      expect(test.stdout, emitsThrough(contains('+1: All tests passed!')));
+      await test.shouldExit(0);
+    });
+
+    test('handles empty lines in the file', () async {
+      await d.file('test.dart', _success).create();
+      await d.file('mytestargs.txt', '\n\r\ntest.dart\r\n').create();
+      var test = await runTest(['@mytestargs.txt']);
+      expect(test.stdout, emitsThrough(contains('+1: All tests passed!')));
+      await test.shouldExit(0);
+    });
+
+    test('handles other arguments in the file', () async {
+      await d.file('mytestargs.txt', '--help').create();
+      var test = await runTest(['@mytestargs.txt']);
+      expectStdoutEquals(test, '''
+Runs tests in this package.
+
+$_usage''');
+      await test.shouldExit(0);
+    });
+  });
 }

pkgs/test_core/lib/src/executable.dart

@@ -68,6 +68,29 @@ Future<void> _execute(List<String> args) async {
           : StreamGroup.merge(
               [ProcessSignal.sigterm.watch(), ProcessSignal.sigint.watch()]);
 
+  // The last arg can start with @ and is a file that contains a list
+  // of args that should replace the filename as if they were just provided
+  // inline.
+  // It is up to the calling code to clean up this file (if required) after
+  // the test process terminates.
+  if (args.isNotEmpty) {
+    args = args.toList(); // Ensure mutable
+    var lastArg = args.last;
+    if (lastArg.startsWith('@')) {
+      lastArg = lastArg.substring(1); // Strip @
+      var argsFile = File(lastArg); // Path must be relative to cwd
+      try {
+        args.removeLast();
+        args.addAll(
+            argsFile.readAsLinesSync().where((line) => line.trim().isNotEmpty));
+      } catch (e) {
+        _printUsage('Failed to read the file "$lastArg" for arguments: $e');
+        exitCode = exit_codes.usage;
+        return;
+      }
+    }
+  }
+
   Configuration configuration;
   try {
     configuration = Configuration.parse(args);